The synchronize action plugin has a built-in list of connection
plugins that it knows how to handle.
One of those connection plugins is the docker connection plugin. And
because the docker content has been moved around quite a lot, the
docker connection plugin has quite a few names:
- docker in Ansible 2.9,
- community.general.docker for community.general < 2.0.0, and
- community.docker.docker since a few months ago.
And while the synchronize module already knew about the first two
names, the last one was still missing. This commit fixes that omission
and adds a third name into the mix.
