Add support for Dockerfile Language Server

[pbnj]

Name: Dockerfile Language Server URL: https://github.com/rcjsuen/dockerfile-language-server-nodejs

Currently, ALE supports hadolint and dockerfile_lint as linters for Dockerfile.

Dockerfile language server provides code completion, hover, diagnostics, and jump to definition features for Dockerfiles.

[pbnj]

Temporary workaround: users can define custom linters in their ~/.vimrc, like so:

call ale#linter#Define('dockerfile', {
      \ 'name': 'docker-language-server',
      \ 'lsp': 'stdio',
      \ 'executable': 'docker-langserver',
      \ 'command': '%e --stdio',
      \ 'language': 'dockerfile',
      \ 'project_root': { _ -> expand('%p:h') }
      \})