* Don't attempt to publish docker images on forks

When the Main CI workflow runs on a fork, the docker push step will
always fail because the appropriate secrets are missing. This is
annoying at best and causes CI on forks to be untrustworthy at worst.
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Use single quotes for string, as github expects
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>