Allow vim be case insensitive for D in dockerfile

Signed-off-by: Kunal Tyagi <tyagi.kunal@live.com>

contrib/syntax/vim/ftdetect/dockerfile.vim

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