docs: Add note about requirement of Docker>=1.0

The built images don't appear to work with Docker<0.10.0, in particular
not with the version of Docker that currently ships as "docker.io" in
the Ubuntu 14.04 repositories.

The install instructions do link directly to the official install docs
for Docker, which don't recommend the distro packages, but we make this
explicit now in a callout at the top of the page.

doc/maintaining/installing/install-using-docker.rst

@@ -2,6 +2,10 @@
 Installing CKAN using a Docker image
 ====================================
 
+.. important::
+   These instructions require Docker >=1.0. The released version of Docker is
+   1.0.1 as at this writing.
+
 .. note::
    Installing CKAN using a Docker image is currently under evaluation. There may
    be omissions or inaccuracies in this documentation. Proceed with caution.