Use a regexp rather than a partial match to skip any DNS_CHECK_OPTIONS,
which in the current tests create whitespace, but could be anything.

Catch a missing inclusion of DNS_CHECK_OPTIONS for dig CNAME

Fixes #594 #272 #564

Fixes #612

Reference:
- https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#cmd

> The `CMD` instruction should be used to run the software contained in
your image, along with any arguments. `CMD` should almost always be used
in the form of `CMD ["executable", "param1", "param2"…]`

This change will drop the history of the repository when cloning, which
will save bandwidth and disk space, speed up the clone and checkout
process during the test and Docker build process.