GH Actions recently changed their Docker build implementation and it
has a different output than previously, causing the tests that parse
its output to fail.
This switches the test to not parse Docker build output. The parsing
was used to extract the image ID while still showing logs. A better
way to show logs and still know the image ID is to tag it, which is
what this now does.
This also renames the Docker tests so that they run earlier. This
takes better advantage of the fact that the sharness tests are run in
parallel. Since the Docker test are quite long, and are at the end of
the list, the test runner is not running other tests in parallel while
the Docker tests are running.
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