Enable TCP port 2375 for external connection to Docker

See this issue.
Docker best practise to Control and configure Docker with systemd.

1. Create daemon.json file in /etc/docker:

 {"hosts": ["tcp://0.0.0.0:2375", "unix:///var/run/docker.sock"]}

2. Add /etc/systemd/system/docker.service.d/override.conf

 [Service]
 ExecStart=
 ExecStart=/usr/bin/dockerd

3. Reload the systemd daemon:

 systemctl daemon-reload

4. Restart docker:

 systemctl restart docker.service