Added coverage for Docker GHA

[radarhere]

Resolves #4212 if this works.

[hugovk]

Thank you, looks good!

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Total coverage with this PR is 87.99%:

• https://codecov.io/gh/python-pillow/Pillow/commit/0893515c98f401cccd0a9940dd6d9a950f49c7d9/build

The same branch, with Docker jobs removed from Travis CI, is also at 87.99%:

• hugovk@ced1098
• https://codecov.io/gh/hugovk/Pillow/commit/ced1098e36940c7f2bd7570981f78d13729127df/build

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I see there's an exception with coveralls-merge on GHA:

coveralls.exception.CoverallsException: Not on TravisCI. You have to provide either repo_token in .coveralls.yml or set the COVERALLS_REPO_TOKEN env var.

• https://github.com/python-pillow/Pillow/pull/4316/checks?check_run_id=367666705

But there's a different exception with coveralls-merge on Travis:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf1 in position 99: invalid continuation byte

• https://travis-ci.org/python-pillow/Pillow/jobs/630891624#L2532

Let's not worry about that for now, at least it's the same end result for both.

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Maybe we keep the Dockers in Travis until January?