Added post on enabling docker TCP socket with Systemd socket activation.

content/docker_tcp_socket.rst

+Bind dockerd to a TCP port
+##########################
+
+:date: 2018-01-08
+:summary: Bind dockerd to a TCP port
+
+On a modern system (one running Systemd) when installing Docker, the
+:code:`dockerd` daemon is run using Systemd' socket activation. By default the
+socket is :code:`/var/run/docker.sock`. If you want to connect to a remote
+machine over TCP, the obvious thing to do is to create
+:code:`/etc/docker/daemon.json` and set the :code:`hosts` list there. But that
+will conflict with the command line flags for socket activation. The correct way
+is to override Systemd' socket activation config. Here's how (all command are as
+:code:`root`):
+
+.. code:: shell
+
+    mkdir -p /etc/systemd/system/docker.socket.d
+    echo '[Socket]' > /etc/systemd/system/docker.socket.d/tcp.conf
+    echo 'ListenStream=2375' >> /etc/systemd/system/docker.socket.d/tcp.conf
+    systemctl daemon-reload
+    systemctl restart docker