container_image may produce invalid tags with recent changes to allowed characters in Bazel labels

As of https://github.com/bazelbuild/bazel/commit/1615da781db78b7a910daf89720189c2b2e73dbe (and I believe an earlier change allowing almost all non-control ASCII characters), a package name may not be a valid Docker image tag.

I believe this stems from https://github.com/bazelbuild/rules_docker/blob/master/container/image.bzl#L367 -- are we comfortable replacing any forbidden characters with `_` or similar? This _does_ mean distinct Bazel targets could have the same image tag, but I am not sure this edge case would be a common occurrence.