Classify Containerfiles as Dockerfiles

[double-daruma]

• When using podman to run an OCI container it is customary to name the Dockerfile as Containerfile instead.
  This is why I propose that Containerfiles should also be classified as Dockerfile by tokei as the two are identical for all practical purposes.
• Similarly podman also supports .containerignore in addition to .dockerignore
• I have not added a test as I was not sure if just copying tests/data/Dockerfile to tests/data/Containerfile would provide any real value to the project

Examples of usage of the term Containerfile in documentation etc.

• https://docs.podman.io/en/stable/markdown/podman-build.1.html
• https://www.redhat.com/en/blog/write-your-first-containerfile-podman