Add docker.socket requirement for docker.service

Without this the docker.socket would not start by default when starting
the docker.service leading to failures to start.

Signed-off-by: Eli Uriegas <eli.uriegas@docker.com>

contrib/init/systemd/docker.service

@@ -4,6 +4,7 @@ Documentation=https://docs.docker.com
 BindsTo=containerd.service
 After=network-online.target firewalld.service
 Wants=network-online.target
+Requires=docker.socket
 
 [Service]
 Type=notify