Running with a user (like `matrix:matrix`) fails if Etherpad is enabled,
because `/matrix/etherpad` is owned by `matrix_etherpad_user_uid`/`matrix_etherpad_user_gid` (`5001:5001`).

The `matrix` user can't acccess the Etherpad directory for this reason
and Borgmatic fails when trying to make a backup.

There may be other things under `/matrix` which similarly use
non-`matrix:matrix` permissions.

Another workaround might have been to add `/matrix/etherpad` (and
potentially other things) to `matrix_backup_borg_location_exclude_patterns`, but:

- that means Etherpad won't be backed up - not great
- only excluding Etherpad may not be enough. There may be other files we
  need to exclude as well

---

Running with `root` is still not enough though.

We need at least the `CAP_DAC_OVERRIDE` capability, or we won't be able to read the
`/etc/borgmatic.d/config.yaml` configuration file (owned by
`matrix:matrix` with `0640` permissions).

---

Additionally, it seems like the backup process tries to write to at least a few directories:
- `/root/.borgmatic`
- `/root/.ssh`
- `/root/.config`

> [Errno 30] Read-only file system: '/root/.borgmatic'
> Error while creating a backup.
> /etc/borgmatic.d/config.yaml: Error running configuration file

We either need to stop mounting the container filesystem as readonly
(remove `--read-only`) or to allow writing via a `tmpfs`.

I've gone the `tmpfs` route which seems to work.

In any case, the mounted source directories (`matrix_backup_borg_location_source_directories`)
are read-only regardless, so our actual source files are protected from unintentional changes.