Add Docker Image reference, to be used for gitlab, docker, etc

[jevanlingen]

What's changed?

Added a new IMAGE reference type, which points to a docker image. Could be used for gitlab-ci files, docker files, testcontainers etc.

Also did a little refactoring in the SourceFileWithReferences:

• Made the class abstract, so the getReferences cache logic can be once instead of every implementation.
• The findMatches method did also check for kind; as the Reference.Matcher does already do that, no need to do it here too.

What's your motivation?

Run one find (and replace?) recipe to search for your docker images in one go, no matter how the image has been configured.

Anything in particular you'd like reviewers to focus on?

Be extra observant, as this is the first time I touch the Reference code. I put most of the code in the rewrite-java package, so maybe that could be improved as well.

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[jevanlingen]

Moved implementation files to

• Improve FindDockerImageUses recipe with multistage files and gitlab-ci files rewrite-docker#9

So in the core it's just the Image reference + all needed things to set it up. In rewrite-docker we have the actual recipe.