Add some test coverage

[pschichtel]

I'm currently considering to migrate a subset jib-gradle-plugin's test over to this plugin, to at least have some smoke testing for releases.

Things to tests:

• Plugin applies on various supported java and gradle versions
• Plugin can build a simple tar
• Plugin can build a docker container (should use a real docker daemon)
• Plugin can publish to a OCI registry (should use a real registry via test-containers)
• Configuration caching in various scenarios
• ???

[pschichtel]

@martinbonnin @sschuberth is there anything in particular you'd like to see tested? The absolute basics are covered now, but I still want to test a few more things people are actually using today.

[sschuberth]

I'd appreciate if all of the issues we ran into so far would be implicitly or explicitly tested to guard against regressions:

• Type 'tel.schich.tinyjib.tasks.TinyJibDockerTask' property 'downloadService' doesn't have a configured value. #10
• Actually do set any configured tags #13
• Version 0.3.2 gives "Serializer for class 'ImageMetadataOutput' is not found" #16

[pschichtel]

Good point.

• Version 0.3.2 gives "Serializer for class 'ImageMetadataOutput' is not found" #16 already gets caught by the current tests in several ways.
• Actually do set any configured tags #13 can be easily added both as a dedicated test and to the existing tests.
• Type 'tel.schich.tinyjib.tasks.TinyJibDockerTask' property 'downloadService' doesn't have a configured value. #10 is more involved, since it requires rather complex projects. It would definitely be valuable to have specific tests for the scenarios where the apply false is required and where it isn't, since it is somewhat fragile