Include Containerfile equivalency (#5588)

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

lib/linguist/languages.yml

@@ -1340,11 +1340,14 @@ DirectX 3D File:
   language_id: 201049282
 Dockerfile:
   type: programming
+  aliases:
+  - Containerfile
   color: "#384d54"
   tm_scope: source.dockerfile
   extensions:
   - ".dockerfile"
   filenames:
+  - Containerfile
   - Dockerfile
   ace_mode: dockerfile
   codemirror_mode: dockerfile