synapse-usage-exporter is self-built by default (no upstream image is
published), so every user enabling it runs the git clone/update task. A
docker-src checkout whose files are owned by a different user (e.g. left
behind by an earlier clone) made that task fail: either with a
permission error, or with git's dubious-ownership protection, which
ignores the repository's own configuration and surfaces as a confusing
"'origin' does not appear to be a git repository" error.

Ensure the checkout's ownership recursively before updating it, and mark
the path as a safe.directory for the git invocation itself (via
GIT_CONFIG_* environment variables), so the ownership check cannot
misfire regardless of which user git effectively runs as.

Fixes #5065

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>