scripts: add helper script to use Docker containers

scripts: add helper script to use Docker containers

This adds a script that can be used to do builds easily within the
defined containers under the automation directory. These containers live
in the public GitLab registry under the xen-project namespace. The
script can be executed a number of ways but the default is to drop you
at a bash shell within a Debian Stretch container at the top level of
the source tree.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>