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Docker

User defined HCL functions

User defined HCL functions

Using interpolation to tag an image with the git sha

As shown in the File definition page, bake supports variable blocks which are assigned to matching environment variables or default values: 

# docker-bake.hcl
variable "TAG" {
  default = "latest"
}

group "default" {
  targets = ["webapp"]
}

target "webapp" {
  tags = ["docker.io/username/webapp:${TAG}"]
}

alternatively, in json format: 

{
  "variable": {
    "TAG": {
      "default": "latest"
    }
  },
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "tags": ["docker.io/username/webapp:${TAG}"]
    }
  }
}

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": [
        "docker.io/username/webapp:latest"
      ]
    }
  }
}

$ TAG=$(git rev-parse --short HEAD) docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": [
        "docker.io/username/webapp:985e9e9"
      ]
    }
  }
}

Using the add function

You can use go-cty stdlib functions. Here we are using the add function. 

# docker-bake.hcl
variable "TAG" {
  default = "latest"
}

group "default" {
  targets = ["webapp"]
}

target "webapp" {
  args = {
    buildno = "${add(123, 1)}"
  }
}

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "buildno": "124"
      }
    }
  }
}

Defining an increment function

It also supports user defined functions. The following example defines a simple an increment function. 

# docker-bake.hcl
function "increment" {
  params = [number]
  result = number + 1
}

group "default" {
  targets = ["webapp"]
}

target "webapp" {
  args = {
    buildno = "${increment(123)}"
  }
}

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "buildno": "124"
      }
    }
  }
}

Only adding tags if a variable is not empty using an notequal

Here we are using the conditional notequal function which is just for symmetry with the equal one. 

# docker-bake.hcl
variable "TAG" {default="" }

group "default" {
  targets = [
    "webapp",
  ]
}

target "webapp" {
  context="."
  dockerfile="Dockerfile"
  tags = [
    "my-image:latest",
    notequal("",TAG) ? "my-image:${TAG}": "",
  ]
}

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": [
        "my-image:latest"
      ]
    }
  }
}

Using variables in functions

You can refer variables to other variables like the target blocks can. Stdlib functions can also be called but user functions can’t at the moment. 

# docker-bake.hcl
variable "REPO" {
  default = "user/repo"
}

function "tag" {
  params = [tag]
  result = ["${REPO}:${tag}"]
}

target "webapp" {
  tags = tag("v1")
}

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": [
        "user/repo:v1"
      ]
    }
  }
}

Using typed variables

Non-string variables are also accepted. The value passed with env is parsed into suitable type first. 

# docker-bake.hcl
variable "FOO" {
  default = 3
}

variable "IS_FOO" {
  default = true
}

target "app" {
  args = {
    v1 = FOO > 5 ? "higher" : "lower" 
    v2 = IS_FOO ? "yes" : "no"
  }
}

$ docker buildx bake --print app

{
  "group": {
    "default": {
      "targets": [
        "app"
      ]
    }
  },
  "target": {
    "app": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "v1": "lower",
        "v2": "yes"
      }
    }
  }
}

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Color output controls

Color output controls

BuildKit and Buildx have support for modifying the colors that are used to output information to the terminal. You can set the environment variable BUILDKIT_COLORS to something like run=123,20,245:error=yellow:cancel=blue:warning=white to set the colors that you would like to use:

Progress output custom colors

Setting NO_COLOR to anything will disable any colorized output as recommended by no-color.org:

Progress output no color

Note

Parsing errors will be reported but ignored. This will result in default color values being used where needed.

See also the list of pre-defined colors.

Read article

Configure BuildKit

Configure BuildKit

If you create a docker-container or kubernetes builder with Buildx, you can apply a custom BuildKit configuration by passing the --config flag to the docker buildx create command.

Registry mirror

You can define a registry mirror to use for your builds. Doing so redirects BuildKit to pull images from a different hostname. The following steps exemplify defining a mirror for docker.io (Docker Hub) to mirror.gcr.io .

  1. Create a TOML at /etc/buildkitd.toml with the following content: 

    debug = true
[registry."docker.io"]
  mirrors = ["mirror.gcr.io"]

    Note

    debug = true turns on debug requests in the BuildKit daemon, which logs a message that shows when a mirror is being used.

  2. Create a docker-container builder that uses this BuildKit configuration: 

    $ docker buildx create --use --bootstrap \
  --name mybuilder \
  --driver docker-container \
  --config /etc/buildkitd.toml

  3. Build an image: 

    docker buildx build --load . -f - <<EOF
FROM alpine
RUN echo "hello world"
EOF

The BuildKit logs for this builder now shows that it uses the GCR mirror. You can tell by the fact that the response messages include the x-goog-* HTTP headers. 

$ docker logs buildx_buildkit_mybuilder0

...
time="2022-02-06T17:47:48Z" level=debug msg="do request" request.header.accept="application/vnd.docker.container.image.v1+json, */*" request.header.user-agent=containerd/1.5.8+unknown request.method=GET spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="fetch response received" response.header.accept-ranges=bytes response.header.age=1356 response.header.alt-svc="h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000,h3-Q050=\":443\"; ma=2592000,h3-Q046=\":443\"; ma=2592000,h3-Q043=\":443\"; ma=2592000,quic=\":443\"; ma=2592000; v=\"46,43\"" response.header.cache-control="public, max-age=3600" response.header.content-length=1469 response.header.content-type=application/octet-stream response.header.date="Sun, 06 Feb 2022 17:25:17 GMT" response.header.etag="\"774380abda8f4eae9a149e5d5d3efc83\"" response.header.expires="Sun, 06 Feb 2022 18:25:17 GMT" response.header.last-modified="Wed, 24 Nov 2021 21:07:57 GMT" response.header.server=UploadServer response.header.x-goog-generation=1637788077652182 response.header.x-goog-hash="crc32c=V3DSrg==" response.header.x-goog-hash.1="md5=d0OAq9qPTq6aFJ5dXT78gw==" response.header.x-goog-metageneration=1 response.header.x-goog-storage-class=STANDARD response.header.x-goog-stored-content-encoding=identity response.header.x-goog-stored-content-length=1469 response.header.x-guploader-uploadid=ADPycduqQipVAXc3tzXmTzKQ2gTT6CV736B2J628smtD1iDytEyiYCgvvdD8zz9BT1J1sASUq9pW_ctUyC4B-v2jvhIxnZTlKg response.status="200 OK" spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="fetch response received" response.header.accept-ranges=bytes response.header.age=760 response.header.alt-svc="h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000,h3-Q050=\":443\"; ma=2592000,h3-Q046=\":443\"; ma=2592000,h3-Q043=\":443\"; ma=2592000,quic=\":443\"; ma=2592000; v=\"46,43\"" response.header.cache-control="public, max-age=3600" response.header.content-length=1471 response.header.content-type=application/octet-stream response.header.date="Sun, 06 Feb 2022 17:35:13 GMT" response.header.etag="\"35d688bd15327daafcdb4d4395e616a8\"" response.header.expires="Sun, 06 Feb 2022 18:35:13 GMT" response.header.last-modified="Wed, 24 Nov 2021 21:07:12 GMT" response.header.server=UploadServer response.header.x-goog-generation=1637788032100793 response.header.x-goog-hash="crc32c=aWgRjA==" response.header.x-goog-hash.1="md5=NdaIvRUyfar8201DleYWqA==" response.header.x-goog-metageneration=1 response.header.x-goog-storage-class=STANDARD response.header.x-goog-stored-content-encoding=identity response.header.x-goog-stored-content-length=1471 response.header.x-guploader-uploadid=ADPycdtR-gJYwC7yHquIkJWFFG8FovDySvtmRnZBqlO3yVDanBXh_VqKYt400yhuf0XbQ3ZMB9IZV2vlcyHezn_Pu3a1SMMtiw response.status="200 OK" spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="do request" request.header.accept="application/vnd.docker.image.rootfs.diff.tar.gzip, */*" request.header.user-agent=containerd/1.5.8+unknown request.method=GET spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="fetch response received" response.header.accept-ranges=bytes response.header.age=1356 response.header.alt-svc="h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000,h3-Q050=\":443\"; ma=2592000,h3-Q046=\":443\"; ma=2592000,h3-Q043=\":443\"; ma=2592000,quic=\":443\"; ma=2592000; v=\"46,43\"" response.header.cache-control="public, max-age=3600" response.header.content-length=2818413 response.header.content-type=application/octet-stream response.header.date="Sun, 06 Feb 2022 17:25:17 GMT" response.header.etag="\"1d55e7be5a77c4a908ad11bc33ebea1c\"" response.header.expires="Sun, 06 Feb 2022 18:25:17 GMT" response.header.last-modified="Wed, 24 Nov 2021 21:07:06 GMT" response.header.server=UploadServer response.header.x-goog-generation=1637788026431708 response.header.x-goog-hash="crc32c=ZojF+g==" response.header.x-goog-hash.1="md5=HVXnvlp3xKkIrRG8M+vqHA==" response.header.x-goog-metageneration=1 response.header.x-goog-storage-class=STANDARD response.header.x-goog-stored-content-encoding=identity response.header.x-goog-stored-content-length=2818413 response.header.x-guploader-uploadid=ADPycdsebqxiTBJqZ0bv9zBigjFxgQydD2ESZSkKchpE0ILlN9Ibko3C5r4fJTJ4UR9ddp-UBd-2v_4eRpZ8Yo2llW_j4k8WhQ response.status="200 OK" spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
...

Setting registry certificates

If you specify registry certificates in the BuildKit configuration, the daemon copies the files into the container under /etc/buildkit/certs . The following steps show adding a self-signed registry certificate to the BuildKit configuration.

  1. Add the following configuration to /etc/buildkitd.toml : 

    # /etc/buildkitd.toml
debug = true
[registry."myregistry.com"]
  ca=["/etc/certs/myregistry.pem"]
  [[registry."myregistry.com".keypair]]
    key="/etc/certs/myregistry_key.pem"
    cert="/etc/certs/myregistry_cert.pem"

    This tells the builder to push images to the myregistry.com registry using the certificates in the specified location ( /etc/certs ).

  2. Create a docker-container builder that uses this configuration: 

    $ docker buildx create --use --bootstrap \
  --name mybuilder \
  --driver docker-container \
  --config /etc/buildkitd.toml

  3. Inspect the builder’s configuration file ( /etc/buildkit/buildkitd.toml ), it shows that the certificate configuration is now configured in the builder. 

    $ docker exec -it buildx_buildkit_mybuilder0 cat /etc/buildkit/buildkitd.toml

    debug = true

[registry]

  [registry."myregistry.com"]
    ca = ["/etc/buildkit/certs/myregistry.com/myregistry.pem"]

    [[registry."myregistry.com".keypair]]
      cert = "/etc/buildkit/certs/myregistry.com/myregistry_cert.pem"
      key = "/etc/buildkit/certs/myregistry.com/myregistry_key.pem"

  4. Verify that the certificates are inside the container: 

    $ docker exec -it buildx_buildkit_mybuilder0 ls /etc/buildkit/certs/myregistry.com/
myregistry.pem    myregistry_cert.pem   myregistry_key.pem

Now you can push to the registry using this builder, and it will authenticate using the certificates: 

$ docker buildx build --push --tag myregistry.com/myimage:latest .

CNI networking

CNI networking for builders can be useful for dealing with network port contention during concurrent builds. CNI is not yet available in the default BuildKit image. But you can create your own image that includes CNI support.

The following Dockerfile example shows a custom BuildKit image with CNI support. It uses the CNI config for integration tests in BuildKit as an example. Feel free to include your own CNI configuration. 

# syntax=docker/dockerfile:1

ARG BUILDKIT_VERSION=v{{ site.buildkit_version }}
ARG CNI_VERSION=v1.0.1

FROM --platform=$BUILDPLATFORM alpine AS cni-plugins
RUN apk add --no-cache curl
ARG CNI_VERSION
ARG TARGETOS
ARG TARGETARCH
WORKDIR /opt/cni/bin
RUN curl -Ls https://github.com/containernetworking/plugins/releases/download/$CNI_VERSION/cni-plugins-$TARGETOS-$TARGETARCH-$CNI_VERSION.tgz | tar xzv

FROM moby/buildkit:${BUILDKIT_VERSION}
ARG BUILDKIT_VERSION
RUN apk add --no-cache iptables
COPY --from=cni-plugins /opt/cni/bin /opt/cni/bin
ADD https://raw.githubusercontent.com/moby/buildkit/${BUILDKIT_VERSION}/hack/fixtures/cni.json /etc/buildkit/cni.json

Now you can build this image, and create a builder instance from it using the --driver-opt image option: 

$ docker buildx build --tag buildkit-cni:local --load .
$ docker buildx create --use --bootstrap \
  --name mybuilder \
  --driver docker-container \
  --driver-opt "image=buildkit-cni:local" \
  --buildkitd-flags "--oci-worker-net=cni"

Resource limiting

Max parallelism

You can limit the parallelism of the BuildKit solver, which is particularly useful for low-powered machines, using a BuildKit configuration while creating a builder with the --config flags. 

# /etc/buildkitd.toml
[worker.oci]
  max-parallelism = 4

Now you can create a docker-container builder that will use this BuildKit configuration to limit parallelism. 

$ docker buildx create --use \
  --name mybuilder \
  --driver docker-container \
  --config /etc/buildkitd.toml

TCP connection limit

TCP connections are limited to 4 simultaneous connections per registry for pulling and pushing images, plus one additional connection dedicated to metadata requests. This connection limit prevents your build from getting stuck while pulling images. The dedicated metadata connection helps reduce the overall build time.

More information: moby/buildkit#2259

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Docker driver

Docker driver

The Buildx Docker driver is the default driver. It uses the BuildKit server components built directly into the Docker engine. The Docker driver requires no configuration.

Unlike the other drivers, builders using the Docker driver can’t be manually created. They’re only created automatically from the Docker context.

Images built with the Docker driver are automatically loaded to the local image store.

Synopsis 

# The Docker driver is used by buildx by default
docker buildx build .

It’s not possible to configure which BuildKit version to use, or to pass any additional BuildKit parameters to a builder using the Docker driver. The BuildKit version and parameters are preset by the Docker engine internally.

If you need additional configuration and flexibility, consider using the Docker container driver.

Further reading

For more information on the Docker driver, see the buildx reference.

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High-level builds with Bake

High-level builds with Bake

This command is experimental.

The design of bake is in early stages, and we are looking for feedback from users.

Buildx also aims to provide support for high-level build concepts that go beyond invoking a single build command. We want to support building all the images in your application together and let the users define project specific reusable build flows that can then be easily invoked by anyone.

BuildKit efficiently handles multiple concurrent build requests and de-duplicating work. The build commands can be combined with general-purpose command runners (for example, make ). However, these tools generally invoke builds in sequence and therefore cannot leverage the full potential of BuildKit parallelization, or combine BuildKit’s output for the user. For this use case, we have added a command called docker buildx bake .

The bake command supports building images from HCL, JSON and Compose files. This is similar to docker compose build , but allowing all the services to be built concurrently as part of a single request. If multiple files are specified they are all read and configurations are combined.

We recommend using HCL files as its experience is more aligned with buildx UX and also allows better code reuse, different target groups and extended features.

Next steps

  • File definition
  • Configuring builds
  • User defined HCL functions
  • Defining additional build contexts and linking targets
  • Building from Compose file
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Create a base image

Create a base image

Most Dockerfiles start from a parent image. If you need to completely control the contents of your image, you might need to create a base image instead. Here’s the difference:

  • A parent image is the image that your image is based on. It refers to the contents of the FROM directive in the Dockerfile. Each subsequent declaration in the Dockerfile modifies this parent image. Most Dockerfiles start from a parent image, rather than a base image. However, the terms are sometimes used interchangeably.

  • A base image has FROM scratch in its Dockerfile.

This topic shows you several ways to create a base image. The specific process will depend heavily on the Linux distribution you want to package. We have some examples below, and you are encouraged to submit pull requests to contribute new ones.

Create a full image using tar

In general, start with a working machine that is running the distribution you’d like to package as a parent image, though that is not required for some tools like Debian’s Debootstrap, which you can also use to build Ubuntu images.

It can be as simple as this to create an Ubuntu parent image: 

$ sudo debootstrap focal focal > /dev/null
$ sudo tar -C focal -c . | docker import - focal

sha256:81ec9a55a92a5618161f68ae691d092bf14d700129093158297b3d01593f4ee3

$ docker run focal cat /etc/lsb-release

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04 LTS"

There are more example scripts for creating parent images in the Docker GitHub repository.

Create a simple parent image using scratch

You can use Docker’s reserved, minimal image, scratch , as a starting point for building containers. Using the scratch “image” signals to the build process that you want the next command in the Dockerfile to be the first filesystem layer in your image.

While scratch appears in Docker’s repository on the hub, you can’t pull it, run it, or tag any image with the name scratch . Instead, you can refer to it in your Dockerfile . For example, to create a minimal container using scratch : 

# syntax=docker/dockerfile:1
FROM scratch
ADD hello /
CMD ["/hello"]

Assuming you built the “hello” executable example by using the source code at https://github.com/docker-library/hello-world, and you compiled it with the -static flag, you can build this Docker image using this docker build command: 

$ docker build --tag hello .

Don’t forget the . character at the end, which sets the build context to the current directory.

Note : Because Docker Desktop for Mac and Docker Desktop for Windows use a Linux VM, you need a Linux binary, rather than a Mac or Windows binary. You can use a Docker container to build it: 

$ docker run --rm -it -v $PWD:/build ubuntu:20.04

container# apt-get update && apt-get install build-essential
container# cd /build
container# gcc -o hello -static hello.c

To run your new image, use the docker run command: 

$ docker run --rm hello

This example creates the hello-world image used in the tutorials. If you want to test it out, you can clone the image repo.

More resources

There are lots of resources available to help you write your Dockerfile .

  • There’s a complete guide to all the instructions available for use in a Dockerfile in the reference section.
  • To help you write a clear, readable, maintainable Dockerfile , we’ve also written a Dockerfile best practices guide.
  • If your goal is to create a new Docker Official Image, read Docker Official Images.
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