Proposal: Build Image Tarball

Motivation

Currently, Jib can build a container image to either a registry or a Docker daemon. Recently, there have been requests for the ability to build an image tarball directly to the filesystem so that the user may load it into a Docker daemon via docker load, either manually or via a build system.

Goals

• Build the image and output as a tarball
• Minimize extra configuration

Constraints

• The tarball will be saved to the project's build output directory with the name jib-image.tar

Intended Workflow

The user will be able to output a docker loadable image tarball by running gradle jibBuildTar for Gradle or mvn jib:buildTar for Maven.

Implementation

The "build tarball" task is almost identical to the "build to docker daemon" task, except for the final step, which writes the final tarball to a file instead of piping it to a docker load command. To avoid duplicate code, a boolean parameter can be used in the steps runners to determine whether to build to a Docker daemon or build to a tarball.

The following changes will be made to the code:

1. Add a private output path configuration parameter to BuildConfiguration
2. Rename BuildTarballAndLoadDockerStep to LoadDockerStep
3. Add a new step WriteTarFileStep for writing a Blob to disk
4. Add BuildSteps#forBuildToTarball(), which would contain the same steps as forBuildToDockerDaemon() up until after the FinalizingStep, where it would run a WriteTarFileStep instead of a LoadDockerStep
5. Add a new task and mojo that would call BuildSteps.forBuildToTarball()