Nice.
I think we could add this new filetype and then update the wiki page to have it only a seperate docker-compose filetype and a reference to the GIT version of this Docker filetype. I'm happy to do this after merge.
Regarding the `extension=` setting: what do you think about using lower case? So that new files will be named "untitled.dockerfile". I don't know if there is a correct answer at all but IIRC what I've seen when there are mutliple dockerfiles is rather something like "Dockerfile.dev" or "Dockerfile.production" but we don't support this ordering in Geany. So, when using extensions, they are usually written in lowercase in my experience.
For the reasons outlined above, I would add to the filetype extension list the pattern "Dockerfile.*" as this is more common than the others in my experience.
Do we need to adjust anything for the Meson build system?