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.
docs: Explicitly note omission of DataStore/DataPusher

Support for these parts of CKAN will come in a future PR.