docs: docker: Crash course on installation

* Nothing less nothing more.

docs/docker.md

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+# Install Latest Docker Service
+
+Docker included with some distributions lags far behind upstream.  This guide aims to provide a quick and reliable way to install or update it.
+
+It is recommended to use platforms that support systemd as future versions of this docker image may require systemd to help with some tasks:
+
+* Fedora
+* Debian 8.1+
+
+## Debian / Ubuntu
+
+### Step 1 — Set Up Docker
+
+Docker is moving fast and Debian / Ubuntu's long term support (LTS) policy doesn't keep up. To work around this we'll install a PPA that will get us the latest version of Docker.
+
+Add the upstream Docker repository package signing key. The apt-key command uses elevated privileges via sudo, so a password prompt for the user's password may appear:
+
+    curl https://get.docker.io/gpg | sudo apt-key add -
+
+Add the upstream Docker repository to the system list:
+
+    echo deb http://get.docker.io/ubuntu docker main | sudo tee /etc/apt/sources.list.d/docker.list
+
+Update the package list and install the Docker package:
+
+    sudo apt-get update && sudo apt-get install -y lxc-docker
+
+Add your user to the `docker` group to enable communication with the Docker daemon as a normal user, where `$USER` is your username. Exit and log in again for the new group to take effect:
+
+    sudo usermod -aG docker $USER
+
+After **re-logging in** verify the group membership using the id command. The expected response should include docker like the following example:
+
+    uid=1001(test0) gid=1001(test0) groups=1001(test0),27(sudo),999(docker)
+
+### Step 2 — Test Docker
+
+Run a Debian jessie docker container:
+
+    docker run --rm -it debian:jessie bash -l
+
+Once inside the container you'll see the `root@<container id>:/#` prompt signifying that the current shell is in a Docker container. To confirm that it's different from the host, check the version of Debian running in the container:
+
+    cat /etc/issue.net
+
+Expected result:
+
+    Debian GNU/Linux jessie/sid