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.
Previously 19.0.3.1 would match multiple versions:
```
5:19.03.11~3-0~debian-stretch
5:19.03.10~3-0~debian-stretch
5:19.03.1~3-0~debian-stretch
```
We can tighten up the regexp by adding a `~` to the end of
the version.
