Allow user to build image as tar file that can be 'docker load'ed

[TadCordle]

https://github.com/GoogleContainerTools/jib/blob/master/proposals/build_tarball.md

• Add private output path configuration parameter
• Rename BuildTarballAndLoadDockerStep to LoadDockerStep
• Add WriteTarFileStep
• Add BuildSteps#forBuildToTarball()
• Add new task to Gradle plugin
• Add new mojo to Maven plugin

[briandealwis]

Would it make sense to attach the tarball as an artifact to the project?

Is there any value in supporting a new packaging that would:

• use package to generate the tar ball
• use install to build to docker
• use deploy to push to registry

[TadCordle]

I'm not sure we want to do that, since currently when we generate the tarball blob, we never actually write it to disk. Adding the tarball as an actual artifact would add some unnecessary I/O to the other steps.

[briandealwis]

Most of the repository managers are adding support for docker images anyways.