This adds the first syntaxchecker for `Dockerfile`. This uses
[dockerfile-lint](https://github.com/projectatomic/dockerfile_lint)
and expect the `dockerfile-lint` binary to be available (usually
through the `npm` install).
By default, it will simply check for basic syntax correctness of the
`Dockerfile`. One can pass a custom `yml` rule file through the `syntastic_dockerfile_dockerfile_lint_post_args`, for example:
```vim
let b:syntastic_dockerfile_dockerfile_lint_post_args = '-r /path/to/rule/file.yml'
```
I've used `preprocess` to convert the json output.
