With glibc 2.34 and the [removal of libpthread](https://developers.redhat.com/articles/2021/12/17/why-glibc-234-removed-libpthread), binaries built using [Debian bookworm](https://www.debian.org/releases/bookworm/) aren't compatible with older but still supported operating systems like RHEL8. In those environments, Mattermost fails to start with errors like:
```
mattermost/bin/mattermost: /lib64/libc.so.6: version `GLIBC_2.32' not found (required by mattermost/bin/mattermost)
mattermost/bin/mattermost: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by mattermost/bin/mattermost)
```
One option might be to generate a static build and avoid the glibc dependency, but this kind of change is out of scope for now. Let's just revert back to using [Debian bullseye](https://www.debian.org/releases/bullseye/), which remains supported until at least August 2026.
(cherry picked from commit c2120b7224ce8d2f8dd34f17fbeba06296b733db)
Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
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