No warning for error "ENV must have two arguments"

[lucasdf]

When declaring an ENV variable its value must be declared too.

When running the linter for the following content of a Dockerfile on earlier versions of dockerlint I received the error ERROR: ENV invalid format ENV_VARIABLE. However, the same error is not displayed anymore since the release 0.3.6

FROM  someimage
ENV  ENV_VARIABLE

ENV requires to be followed by both a key and a value. When trying to build a Dockerfile containing the content above, Docker displays Error response from daemon: ENV must have two arguments.

So, it seems that dockerlint should go back to the earlier behavior and display an error message in such situation.

Related to #46

[lucasdf]

awesome! thanks!