Keep a site's own files when its container is replaced

`db_data` was a volume and the instance directory was not, so the documented
update path - `docker compose build api && up -d api` - recreated the container
and discarded everything the site had written. `plugins.json` is only the loud
part: maps, branding, model and application images, employee photos, warranty
proofs, slides, printed-part files and the Dell OAuth token all live under
instance_path too. MySQL rows survive and point at files that are gone, so the
second symptom is images 404ing rather than an error anybody sees.

Reported by an adopting site, which read it as having updated too fast. It had
not; nothing it could have done differently would have kept those files.

DEPLOY.md had been telling sites to back up `instance/` since it was written.
The template never gave them anything to back up.

The air-gap `migrate` service mounts the volume too, because
`flask plugin upgrade-all` rewrites plugins.json and that service exits
immediately after.

The image now creates instance/ ITSELF, owned by the app user. Docker seeds an
empty named volume from image content at the mountpoint, ownership included;
with no such directory in the image the mountpoint is created root-owned 0755
and the container, which runs as shopdb, cannot write into its own instance
directory. Caught by running the built image rather than by reading it: the
volume mounted clean and `touch` came back Permission denied. Verified fixed the
same way.

A stack that predates the volume needs its files moved across ONCE, while the
old container still exists - the volume is seeded from image content, and the
image ships instance/ empty, so it comes up empty rather than inheriting the old
container's writable layer. DEPLOY.md carries the procedure, including the chown
after `docker compose cp`, which writes files under the copying user's numeric
uid rather than the app user's.

Also here, found while checking what an upgrade actually runs: the connected
update steps ran `flask db upgrade` and stopped. Per-plugin Alembic chains
(ADR-008) are not part of that, so a connected site taking an image with a
bumped plugin migration ran the core chain and silently skipped every plugin
chain. The air-gap stack had it right all along. Both commands are in Step 9
now, plus a `db current` check against `db heads`.

Dockerfile

@@ -72,6 +72,13 @@ COPY wsgi.py ./
 # Built SPA from stage 1. Flask serves it via register_frontend_routes.
 COPY --from=frontendbuild /build/frontend/dist ./frontend/dist
 
+# Create instance/ IN THE IMAGE, owned by the app user, before the chown below.
+# Docker seeds an empty named volume from the image's content at the mountpoint,
+# ownership included. Without this the mountpoint is created root-owned 0755 and
+# the container, which runs as shopdb, cannot write plugins.json or any upload
+# into its own instance directory.
+RUN mkdir -p /app/instance
+
 RUN useradd --create-home --shell /bin/bash shopdb \
     && chown -R shopdb:shopdb /app
 USER shopdb