This was previously a mirror of github.com/docker/docker/contrib/syntax/vim, but in docker/docker@5511f45, that entire directory was deprecated in favor of ftplugin/dockerfile.vim and syntax/dockerfile.vim in github.com/vim/vim, which is then also part of the popular Vim fork, github.com/neovim/neovim.
The docker branch contains the old Docker-based commit history.
The neovim branch and the vim branch contain the new (Neo)Vim-based commit history (and are now the useful ones to include/use via vim-pack, etc; compare vim..neovim).
The tianon branch contains my personal fork of the vim branch with some minor changes (which I'd normally try to contribute back upstream, but ...).