Include Containerfile equivalency

[cevich]

Description

This file is exactly equivalent in every way and context. Exact same highlighting rules and handling should be provided for either name style.

Checklist:

• I am associating a language with a new filename.
  • The new filename is used in hundreds of repositories on GitHub.com:
    ~1,640 search results

[cevich]

We generally only add aliases when a language has multiple possible spellings, or is known by more than one name. Unless the name "Dockerfile" can be used interchangeably with "Containerfile" (and vice versa, regardless of context), I suggest leaving it out.

Actually yes, a Containerfile and a Dockerfile are 100% equivalent in every respect, and in every context. It's simply a de-branded name for the exact same underlying file-type AND contents.

In other words, the goal is for (in every context), these to be the same:

```Dockerfile
...blah...blah...blah...
```

and

```Containerfile
...blah...blah...blah...
```

Similarly when viewing the contents of a repository file or diff referencing either a Containerfile or a Dockerfile. They should look/feel/act exactly equivalent.

[cevich]

Updates: Fixed up the description to match commit message + added similar equivalency for .containerfile.

[cevich]

Update: Added .containerfile to the list as well.

[cevich]

Update: Removed .containerfile and .containerignore references, updated commit message and PR title + description.

[cevich]

Thanks for your help and guidance @Alhadis