added missing alias to docker doc fragment

lib/ansible/utils/module_docs_fragments/docker.py

@@ -26,6 +26,7 @@ options:
             - URL or Unix socket path used to connect to the Docker daemon.
         required: false
         default: null
+        aliases: ['docker_url']
     tls_hostname:
         description:
             - If verifying the name of the host found in the TLS certs, provide the expected host name.
@@ -76,4 +77,4 @@ notes:
       DOCKER_TLS, DOCKER_TLS_VERIFY and DOCKER_TIMEOUT. If you are using docker machine, run the script shipped
       with the product that sets up the environment. It will set these variables for you. See
       https://docker-py.readthedocs.org/en/stable/machine/ for more details.
-'''
+'''