support *.Dockerfile

Probably a good idea to treat `*.Dockerfile` as dockerfile format as well. In general, it's better to use the `Dockerfile` part as an extension rather than a basename.

Signed-off-by: Andrew Pennebaker <andrew.pennebaker@gmail.com>

contrib/syntax/vim/ftdetect/dockerfile.vim

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