Allowing to specify Bazel label for docker config.json in container_push

[linzhp]

🚀 feature request

Relevant Rules

• container_push

Description

Currently, if people wants to use a custom config.json for container_push, they have to specify the absolute path to the directory, which is machine-dependent. #2032 allows container_pull to use a Bazel label to config.json. We need to make container_push to have that too.

Describe the solution you'd like

Technically, it's wrong for any Bazel rule to take an absolute path as input. Since rules_docker is not v1.0 yet, I wish we could introduce a breaking change to make all client_config and docker_client_config attributes to be Bazel label only.

Describe alternatives you've considered

We can continue to accept both absolute paths and Bazel labels.

[linzhp]

@gravypod @alexeagle Can you review #2041 and #2042, and advice which is the right way to go?