Add ONCE backup and restore hooks

By default, ONCE pauses the application container when taking a backup.
This guarantees a consistent snapshot of the data, but for a large
installation and/or a slow destination path (like a network mount), that
pause could be a bit disruptive to live traffic.

But ONCE also support a pause-free path: if a `pre-backup` hook is
present in the container, ONCE will run that and skip the pause. So by
supplying necessary hooks, we can avoid any disruption from backups.

- `pre-backup` runs the existing script/admin/prepare-backup, which uses
  SQLite's online backup API to get a consistent snapshop. k

- `post-restore` moves our snaphotted file back into place, and removes
  the orphaned sidecar files

Dockerfile

@@ -66,6 +66,9 @@ ENV HTTP_WRITE_TIMEOUT=300
 COPY --from=build --chown=rails:rails /usr/local/bundle /usr/local/bundle
 COPY --from=build --chown=rails:rails /rails /rails
 
+# Install ONCE backup/restore hooks
+COPY --chmod=755 hooks /hooks
+
 # Set version and revision
 ARG APP_VERSION
 ENV APP_VERSION=$APP_VERSION