script/build-with-container: support custom tag suffixes

script/build-with-container: support custom tag suffixes

Previously, one could use the `--tag` option to completely override the
container tag generated by the script. However, there are cases where
one may want to add information to the tag rather than override it.
Allow the tag value to start with a plus (+) character that indicates
that the remainder of the string is to be suffixed to the generated tag.

Signed-off-by: John Mulligan <jmulligan@redhat.com>