website: start documenting the builder

website/source/docs/builders/docker.html.markdown

+---
+layout: "docs"
+---
+
+# Docker Builder
+
+Type: `docker`
+
+The Docker builder builds [Docker](http://www.docker.io) images using
+Docker. The builder starts a Docker container, runs provisioners within
+this container, then exports the container for re-use.
+
+The Docker builder must run on a machine that supports Docker.
+
+## Basic Example
+
+Below is a fully functioning example. It doesn't do anything useful, since
+no provisioners are defined, but it will effectively repackage an image.
+
+<pre class="prettyprint">
+{
+  "type": "docker",
+  "image": "ubuntu",
+  "export_path": "image.tar"
+}
+</pre>
+
+## Configuration Reference
+
+All configuration options are currently required.
+
+* `export_path` (string) - The path where the final container will be exported
+  as a tar file.
+
+* `image` (string) - The base image for the Docker container that will
+  be started. This image will be pulled from the Docker registry if it
+  doesn't already exist.
+
+## Dockerfiles
+
+TODO

website/source/layouts/docs.erb

@@ -32,6 +32,7 @@
 			<li><h4>Builders</h4></li>
 			<li><a href="/docs/builders/amazon.html">Amazon EC2 (AMI)</a></li>
 			<li><a href="/docs/builders/digitalocean.html">DigitalOcean</a></li>
+			<li><a href="/docs/builders/docker.html">Docker</a></li>
 			<li><a href="/docs/builders/openstack.html">OpenStack</a></li>
 			<li><a href="/docs/builders/qemu.html">QEMU</a></li>
 			<li><a href="/docs/builders/virtualbox.html">VirtualBox</a></li>