Fix Docker Compose ftdetect

[eligundry]

This upgrade simplifies the Docker Compose file type detection down to
one regex. It also prevents that ftdetect from detecting itself.
Finally, it adds support for .yaml extensions and drops support for
docker-compose files without an extension, as that is madness.

I also changed all the setfs to set ft because it wasn't working in
Neovim for some reason.

[ekalinin]

Thanks!