Support testing with PostgreSQL

Support testing with PostgreSQL

This allows us to easily test against PostgreSQL using the same
tooling we normally use. This is helpful in (for example) shaking
out the test failures that were observed on ozlabs.org

To use it:
  docker-compose -f docker-compose-pg.yml <usual argument>

(You may find in necessary to do a 'docker-compose down' first,
depending on what state the system is in and what command you're
running.)

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>