Integration testing with Docker and testcontainers

[andregasser]

I was able to get a specific Mastodon instance version running in Docker, using a docker-compose.yml. I was basically following the guide published here: https://www.raeffs.dev/blog/self-hosting/2022-11-11-mastodon-with-docker/

My current plan is to build some sort of JUnit5-based integration tests that use testcontainers (https://https://www.testcontainers.org/) with the Docker compose module (https://www.testcontainers.org/modules/docker_compose/) to spin up specific versions of Mastodon (e.g. 3.5.0, 4.0.2, ...), invoke some library calls against that instance to see whether our request pass / fail. Doing so, we could easily verify, whether our calls are really working fine in a close-to-realistic environment.

I see some nice advantages when doing this:

• Tests are portable to other systems, as no local software must be installed to run the tests
• Mastodon instances are under our control
• We could execute specific tests (testing error conditions and such), that would otherwise be hard to do
• We can execute the tests from the IDE easily (hopefully), as it is just a JUnit test
• It helps to identity potential breaking changes that we have missed

I think it would be cool if we coudl build a first version of a JUnit 5 test class that invokes some API calls
against the latest Mastodon 4.0.2 instance.

In the beginning, we could run those tests manual for verification purposes, later we could add them as a GitHub check, as soon as they are stable enough.

Another nice side effect of having Mastodon instances running in Docker is that we can use these to extract real API responses for our existing tests.