Add .containerfile extension to Dockerfile

[lildude]

Description

Add .containerfile to the Dockerfile language.

Closes #6799

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?q=NOT%20is%3Afork%20path%3A*.containerfile&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/saucelabs/forwarder/blob/02c3e9f05eaccb2f59dbca7a5b580fb1639b5980/local/linux/debian-systemd.Containerfile#L4
    • Sample license(s): MPL-2.0