Add some form of tests

[nils-a]

unit tests

Check if it's possible to add some unit tests to https://github.com/nils-org/dependabot-core/tree/cake/main - if so, they should run here before we pack that version into a new image.

integration tests

We need some form of integration tests, so that things like f2b01cb will not happen again.

"layers" that should be covered by integration tests:

• the action, which calls the docker container with environment variables
• the docker image creation (can the image be build, do the scripts run?)
• the "dependabot shim" which gets the parameters from the environment and runs (the Cake) parts of dependabot core