Add support for .containerfile extension

[gnat]

Containerfile is currently an alias for Dockerfile great!

blah.Dockerfile works but blah.Containerfile does not.

[lildude]

If the extension is popular enough to meet our usage requirements, feel free to submit a PR to add support. You can find details in the CONTRIBUTING.md file.

[gnat]

Linguist already supports Containerfile as an alias of Dockerfile 👀

Containerfile is just the standard OCI name

[lildude]

Indeed. But it's not going to magically appear in the languages.yml file without a PR, and support won't be added until usage meets our requirements. This applies to all additions for new and existing languages.