Docker deployment with bazel-defined version tag

[flyingsilverfin]

Problem to Solve

To make a consistent API within this repository, we can create a variant of @io_bazel_rules_docker//container:container.bzl's container_push which instead of taking a tag argument, reads from the defined variables in the context.

We should also allow differentiating the docker-tag version from the standard version tag that is used elsewhere, so that the deployment tag doesn't have to be the exact same as the version number written into the distribution archive.

Current Workaround

We define the docker tag into a VERSION file, which is read into the target in BUILD files.

Proposed Solution

We create a container push rule that allows to define the tag name to deploy against using a --define container-tag=<VERSION> in the invocation of the build rule.