This is mostly here to guard against problems happening
due to server migration and doing `chown -R matrix:matrix /matrix`.
Normally, the file is owned by `1000:1000`, as expected.
If ownership changes, Dimension could still start, but it will fail the
first time it tries to write to the database. Explicitly chowning
before startup guards against this.
Related to #485 and #486 (Github Pull Requests).
Also related to ccc7aaf0ce.
Reasoning is the same as for matrix-org/synapse#5023.
For us, the journal used to contain `docker` for all services, which
is not very helpful when looking at them all together (`journalctl -f`).
