Use GitHub Actions instead of CircleCI

[AlexZeitler]

Why?

• GitHub Actions should be good enough for us
• Getting rid of additional tooling

[AlexZeitler]

While working on this issue, I noticed there are several issues with our tests:

• The alpine services are non-blocking which caused the tests to fail on CircleCI due to timing issues (the alpine services never needed to stop because they did stop on their own due to the lack of a blocking command running in the service). This can be fixed by e.g. using another nginx image instead of alpine.
• The tests for --abort-on-container-exit option aren't able to stop/down the services which causes a timeout. I don't have a solution for this right now. @Steveb-p any ideas?