Merge pull request #13605 from tianon/vim-dockerfile

Adjust vim ftdetect to match "Dockerfile", "dockerfile", and "Dockerfile.*" which are all reasonably safe to assume "this is a Dockerfile"

contrib/syntax/vim/ftdetect/dockerfile.vim

@@ -1 +1 @@
-au BufNewFile,BufRead Dockerfile set filetype=dockerfile
+au BufNewFile,BufRead [Dd]ockerfile,Dockerfile.* set filetype=dockerfile