gitlab-ci: Use Debian stretch instead of Ubuntu bionic

The APT archive used by the Ubuntu docker image can be slow, even timing
out sometimes, causing spurious failures of the containers-build job.
The Debian docker image uses deb.debian.org, which is backed by a
content distribution network.

One downside is that stretch only has GCC 6, whereas bionic had 7.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
2 files changed