Use rocker to push multiple images

After https://github.com/coala/docker-coala-base/issues/226 , there is a good chance that we can use rocker to implement https://github.com/coala/docker-coala-base/issues/203 , a separation between a OS layer image and another layer that includes coala fixes and linter fixes only.

This can be done by pushing the OS layer image during initial release builds, and then using it for patch builds.

Read issues 226 and 203 first, then inspect the repository's release-build and patch-build entry points. Verify how rocker can separate the OS layer from coala and linter fixes, and consider the work complete when initial releases push the OS image and patch builds reuse it.

Written by the indexing model from the issue text.