GHA: Separate container build and run steps to simplify reading workflow logs

[eecavanna]

The python-app.yml workflow currently invokes:

    - name: Test with pytest
      run: |
        make up-test
        make test

That step takes a long time (several minutes) and produces a lot of console output (many scrolls). I find it tedious to scroll through the console output in order to find the portion that shows the test output.

I propose the GHA workflow be updated so that the container building step happen in a different workflow step from the actual running of the tests.