Don't just mount spec folder, but complete app folder for tests

[Splines]

This is to ensure that local changes in the app folder are reflected in the Docker container such that newly written tests can be run through without having to rebuild the whole Docker image.

• While the whole app folder is mounted to /usr/src/app/app, wee construct a specific public volume such that the rake assets:precompile task can populate the folder usr/src/app/public/packs-test/.

• Note that all files are still copied in the Dockerfile such that rails assets:precompile can work correctly. The app/ folder is then later mounted "on top" of that.

• Also moved the entrypoint declaration to the place where it's declared in the development setup, just to achieve symmetry.

For reviewers

Please delete your old mampf test image (if you have one locally) before testing the changes made in this PR.

A good way to test the changes in this PR could be as follows:

• git fetch
• git switch feature/user-cleaner
• git merge tests/mount-app-folder (only for your local playground, so please don't push)
• Check if tests run through locally. Everything should work fine.
• Rename app/models/user_cleaner.rb to user_cleaner_changed.rb (also the class name). Adjust accordingly in user_cleaner_spec.rb. Then, run the tests again. Everything should still work fine. With this procedure, you've checked that the locally changed file content (user_cleaner_changed.rb) is available in the Docker container. If it wasn't, you would get an error saying that there was no class called UserCleanerChanged.
• After this, reset your local git merge: git reset --hard origin/feature/user-cleaner. Note this will hard-reset any modified changes! Instead, you could also do git reset HEAD~1 to keep your modifications (and those by the initial git merge you made).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.63%. Comparing base (4ead9d5) to head (5ec9818).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #648   +/-   ##
=======================================
  Coverage   66.63%   66.63%           
=======================================
  Files         309      309           
  Lines        9384     9384           
=======================================
  Hits         6253     6253           
  Misses       3131     3131           

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