* ci: only spin up a docker engine if necessary

Halt the website-docker-image job early if no changes are detected. We
halt early before spinning up the remote docker engine, as the remote
docker engine step can add some delay (seconds to minutes) and is more
likely to suffer circleci instability.

* ci: Only run website workflow in OSS repo

Add a CircleCI conditional to avoid running website worklows on forks.