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

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

contrib/syntax/vim/ftdetect/dockerfile.vim

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