Fix type annotations for Docker labels

[vivienm]

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
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the changes folder
  • name it <issue_id>.<type> for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[asvetlov]

Thanks!