Corrected first_line_match: only match "from" or "arg" in the beginning of the line

[mitranim]

first_line_match does not have implicit ^$. As a result, the rule was matching things like import blah from 'blah', which occasionally marked non-docker files without explicit file extensions, or in unsaved buffers, as dockerfiles. This PR fixes it.

[princemaple]

made a patch release