Need to document docker image autobuilding

[timlinux]

To set up autobuilds used in the production server:

Go to http://hub.docker.com

Then:

You should see the status panel like this:

In the Hub repo now go and configure it so that updates to the base image trigger a new build:

Also make sure you have set up the tags and pointed to the correct dockerfile.

The build tags should point to :

• Dockerfile.production
• Dockerfile.testing

[Xpirix]

@timlinux , perhaps we can use the same approach as the qgis-plugins-uwsgi image and proposed in the PR https://github.com/qgis/qgis-feed/pull/92