bullseye is the latest stable, and this is what we will be using for
Cloud Native GitLab going forward so we should test with this.
Since an upgrade of the operating system may break pre-compiled C
extensions, we now introduce a DEBIAN environment flag that defaults
to the legacy buster install but allows us to roll out a bullseye
image with `debian-bullseye` as the image name prefix.
Every package manager should clean up after themselves in order to keep
docker layers neat and tiny:
apt (Debian/Ubuntu package manager):
- unneeded dependencies are cleared (autoremove)
- caches are cleaned (clean)
- package lists are deleted
yum (CentOS package manager),
zypper (OpenSuse package manager)
- should clear caches after installing dependencies
pip (Python package manager),
apk (Alpine package manager)
- should use no cache for installing dependencies
