What do these changes do?

This commit fixes invalid type hints for Docker labels: the Docker engine API expects a string-to-string mapping, not a list. See for instance https://docs.docker.com/engine/api/v1.41/#operation/ServiceCreate.

Are there changes in behavior for the user?

Labels require dicts, but Mypy expects lists, so users need to introduce # type: ignore hints when using labels in typechecked code. If this PR gets accepted, theses hints will become spurious and, depending on Mypy settings, users may have to remove them for typechecking to succeed.

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist (note: tests use correct types already)
• Documentation reflects the changes (note: unrelevant)
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
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    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
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  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."