Adds a php conf.d partial that allows setting arbitraty configuration
values.
Enable xdebug remote connect back, so the server tries to connect back
to the requesting host on each request.

Codecoverage in codeception uses the C3 helper script that needs to be
included in the site running. This collects code coverage even
(especially) for acceptance tests, e.g. where codeception itself does
not have control over the code.
For coverage collection in general, XDebug is needed and therefore also
activated.

Production usage not yet possible due to
https://github.com/Codeception/Codeception/issues/3337 also coverage
collection is pretty slow.

codeception db populator loads a database dump between each test so all
tests operate on an indepentent, clean database.

This data can be used to repopulate the test database between all test
runs.

... however it doesn't work. I can manually send mails from app
container successfully, but app does not do so.

I first tried without user/pass and that didn't work either.

Selenium container downloads will be made available to the app to be
able to test output.

As in CI, selenium in test may fail due to memory constraints as well.

There is a web driver error "No such session" occuring quite often in
CI.

https://bugs.chromium.org/p/chromedriver/issues/detail?id=732#c31
suggests this might be a memory related problem.

Previously it would only send websocket messages to the other users
and rely on the return data in the call to create the message. But
this does not allow other sessions/connections of the user to keep
up to date with the websocket messages.