Inform synchronize module about community.docker collection

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.

changelogs/fragments/144_add_community_docker_connection_plugin_alias.yml

@@ -0,0 +1,4 @@
+---
+bugfixes:
+  - synchronize - add ``community.docker.docker`` to the list of supported
+    transports (https://github.com/ansible-collections/ansible.posix/issues/132).