Add integration tests for Docker image

[kaklakariada]

We should add an integration test that verifies exit codes and output variables.

[sophokles73]

Would you imagine this being run as a CI workflow?

[kaklakariada]

Yes. But I can do it myself later, no problem!

[sophokles73]

Yes. But I can do it myself later, no problem!

That would be nice 👍