Add integration test with backend

[henrikhorluck]

Is your feature request related to a problem? Please describe. We sometimes want to do changes/dependency upgrades with out OAuth dependencies on our backend, and it would greatly help to have som integration tests ready, to ensure that e.g. login always works, and Stripe-API as well.

Describe the solution you'd like A Docker image pushed in CI from this repo, preferably one with every release (and start to use git tags for releases), along with a "latest/canary" for latest main-branch, which is run in CI in dotkom/onlineweb4

Describe alternatives you've considered Write the backend tests from here and instead? Maybe both?

Additional context Docker! 🚀