Changing docker client config attributes to Bazel labels

[linzhp]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: #2040

What is the new behavior?

toolchain_configure only takes a Bazel label

Does this PR introduce a breaking change?

• Yes
• No

Existing applications will have to change the client_config of their toolchain_configure rule to be a Bazel label.

Other information

This is more correct than #2032 and #2041, but it introduces a breaking change.

[uhthomas]

The PR and logic itself seems sound to me, I don't see any use-cases where an absolute path would be desirable. Despite this, it should still possible for the toolchain to use the client config specified in the home directory, or with the DOCKER_CONFIG environment variable.|

Pending response to the comments I left, I'm happy to approve this.

[uhthomas]

Thank you for making those changes! LGTM