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OCI Image Builder

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Please test your images (and add to our conformance suite)!

This library supports using the Dockerfile syntax to build OCI & Docker compatible images, without invoking a container build command such as buildah bud or docker build. It is intended to give clients more control over how they build container images, including:

  • Instead of building one layer per line, run all instructions in the same container
  • Set HostConfig settings like network and memory controls that are not available when running container builds
  • Mount external files into the build that are not persisted as part of the final image (i.e. "secrets")
  • If there are no RUN commands in the Dockerfile, the container is created and committed, but never started.

The final image should be 99.9% compatible with regular container builds, but bugs are always possible.

Future goals include:

  • Output OCI compatible images
  • Support other container execution engines, like runc or rkt
  • Better conformance testing
  • Windows support

Install and Run

To download and install the library and the binary, set up a Golang build environment and with GOPATH set run:

$ go install github.com/openshift/imagebuilder/cmd/imagebuilder@latest

The included command line takes one argument, a path to a directory containing a Dockerfile. The -t option can be used to specify an image to tag as:

$ imagebuilder [-t TAG] DIRECTORY

To mount a file into the image for build that will not be present in the final output image, run:

$ imagebuilder --mount ~/secrets/private.key:/etc/keys/private.key path/to/my/code testimage

Any processes in the Dockerfile will have access to /etc/keys/private.key, but that file will not be part of the committed image.

You can also customize which Dockerfile is run, or run multiple Dockerfiles in sequence (the FROM is ignored on later files):

$ imagebuilder -f Dockerfile:Dockerfile.extra .

will build the current directory and combine the first Dockerfile with the second. The FROM in the second image is ignored.

Note that imagebuilder adds the built image to the docker daemon's internal storage. If you use podman you must first pull the image into its local registry:

$ podman pull docker-daemon:<IMAGE>:<TAG> # must contain either a tag or a digest

Code Example

f, err := os.Open("path/to/Dockerfile")
if err != nil {
	return err
}
defer f.Close()

e := builder.NewClientExecutor(o.Client)
e.Out, e.ErrOut = os.Stdout, os.Stderr
e.AllowPull = true
e.Directory = "context/directory"
e.Tag = "name/of-image:and-tag"
e.AuthFn = nil // ... pass a function to retrieve authorization info
e.LogFn = func(format string, args ...interface{}) {
	fmt.Fprintf(e.ErrOut, "--> %s\n", fmt.Sprintf(format, args...))
}

buildErr := e.Build(f, map[string]string{"arg1":"value1"})
if err := e.Cleanup(); err != nil {
	fmt.Fprintf(e.ErrOut, "error: Unable to clean up build: %v\n", err)
}

return buildErr

Example of usage from OpenShift's experimental dockerbuild command with mount secrets

Run conformance tests (very slow):

docker rmi busybox; docker pull busybox
docker rmi alpine; docker pull alpine
docker rmi centos:7; docker pull centos:7
docker rmi registry.fedoraproject.org/fedora-minimal; docker pull registry.fedoraproject.org/fedora-minimal
docker rmi registry.fedoraproject.org/fedora-minimal:41-x86_64; docker pull registry.fedoraproject.org/fedora-minimal:41-x86_64
docker rmi registry.fedoraproject.org/fedora-minimal:41-aarch64; docker pull registry.fedoraproject.org/fedora-minimal:41-aarch64
chmod -R go-w ./dockerclient/testdata
go test ./dockerclient -tags conformance -timeout 30m