dockerlint incorrectly alerts on any commands included within a RUN<<HEREDOC stanza

### Issue details

dockerlint incorrectly reports errors for every line inside a RUN<<HEREDOC ... HEREDOC stanza.

### Steps to reproduce/test case

1. create any Dockerfile
2. add any RUN<<EOF ... EOF stanza

### Affected version(s)

All.  Currently, I'm using the most recent version as of the date of this report  (0.3.2).
