Including tag_opts in the spec builderfile

Unfortunately, this test case still doesn't cover the actual bug, since
that code path is only encountered during real container builds.  At the
moment, the tests don't include real container builds because those are
not supported on Travis.  Putting a note in the TODO file.
Making sure we're actually replacing the right arg when tagging

The tag command has a placeholder "`<IMG>`" value for the image id,
since that is not available until after the build is completed.
After the build command is run, the placeholder is then replaced by the
actual image id...as of this commit.