Add lint rule forbidding single quotes in Dockerfile

[AnalyzePlatypus]

According to the Dockerfile documentation, CMD's array syntax must use double quotes:

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

# Valid
CMD ["/bin/bash", "echo", "this should work"]

# Invalid
CMD ['/bin/bash', 'echo', 'mysteriously fails']

This build will fail with the unintelligible error /bin/sh: 1: /bin/echo,: not found
I just spent a few hours scratching my head, until I found this StackOverflow answer.

Can we include a lint rule that explicitly complains if the wrong form of quotes in used?

[rcjsuen]

Hi, @AnalyzePlatypus. Thank you for the bug report. This is being tracked by rcjsuen/dockerfile-utils#28. I will leave this open until the language service adopts the fix.