travis-ci: Add and use infrastructure to build and test in Docker

travis-ci: Add and use infrastructure to build and test in Docker

Debian stable, Debian testing and Ubuntu LTS provide a reasonable
spectrum of old and new distributions. I'm only doing one build on
each to avoid a combinatorial explosion of options.

The Docker images don't have any deb-src apt sources set up, so don't
use `apt-get build-dep`; just include dependencies manually.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=98889