I've renamed it to filetypes.Dockerfile.conf and now it shows up under Document->filetype->Misc :+1:
but there's no syntax highlighting when I'm viewing a Dockerfile. `lexer_filetype=Dockerfile`
Well thats because there is no Dockerfile lexer, which is because there is not one in the [Lexilla](https://github.com/ScintillaOrg/lexilla/tree/master/lexers) project they come from. A "custom" filetype is really a "customisation" of existing filetypes, so it can only use lexers already in Geany. You might find one that is "good enough" but YMMV.