containers.yml: build with docker.py tooling

Instead of building the docker files directly use the same docker.py
scripting as we do for building locally. This should help ensure we
use the exact same steps and allow us to cache properly when building
locally.

To get this working you have to have a fairly recent docker binary
otherwise you will see the error message:

 => ERROR importing cache manifest from registry.gitlab....

So far docker 19.03.12 works (from the docker apt repos) but 18.09.1,
build 4c52b90 which is packaged in Debian Buster fails.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200701135652.1366-39-alex.bennee@linaro.org>