Add Containerfile file-type for dockerfile language

[TheAlgorythm]

The Containerfile is a vendor independent name for the Dockerfile when using things like podman. It has the same syntax and therefore it is possible to extend the file-types.
But I am not sure whether it would be better to define a different language with the same parameters. It would better show being vendor independent but also a bit more bloat.
Further I am not sure why there is a lowercase dockerfile in the file-types and if it is also needed for the Containerfile.

[the-mikedavis]

.dockerfile is a less common file extension for dockerfiles, for example build.dockerfile. It looks like .containerfile is used by some projects (see https://github.com/search?q=path%3A*.containerfile&type=code) so it probably makes sense to add it here as well

[TheAlgorythm]

In my experience this file extension always started uppercase but adding the lowercase one shouldn't hurt. So I did add it.