Tag use can crash the the unit tests

[roadSurfer]

Change the tag in "test/data/dockerfiles/TestLabels" from "sometag" to "some-tag". Now run the tests with "node bin/dockerfile_lint --json -f test/data/dockerfiles/TestLabels -f test/data/dockerfiles/TestLabels -p -r test/data/rules/basic.yaml"

Observe that they fail with "Invalid parameters for command." on the "FROM" command.

Not quite sure what is going on there, but I would expect "some-tag", "some_tag" etc to all work without issue.

[roadSurfer]

I think this could be related to issue 105