Revert a local: identifier in the test
Revert a local: identifier in the test

Testing tooling can sometimes be finicky. THe work-around to ensuring it
uses the local copy vs the store copy is to export
JUJU_TEST_CHARM='docker' and not hard-code a local: identifier, as this
confuses CI situations where there is not a local checkout of the charm.

chuckbutler/docker-charm,chuckbutler/docker-charm