Stop the SQLite volume from hiding new migrations

Uploads on share.kadenpartner.ch failed with 409 on every chunk after
the update to 2.1. The example docker-compose.yml mounts the SQLite
volume over all of /app/database. Docker fills a volume from the image
only when it is created, so the container kept the 2.0 migrations and
never saw the 2.1 one. share_files.uploaded_chunks was never created,
so the chunk endpoint read null and answered 409. SQLite takes the
unknown "completed_at" in whereNull() as a string, so nothing failed
earlier.

- The image keeps a copy of its migrations in docker/migrations. On
  startup the entrypoint adds the ones missing from database/migrations
  before migrating, so installs with the old mount recover by pulling
  the new image.
- docker-compose.example.yml and docker-compose.yml mount
  sealshare_database at /app/database/sqlite and set DB_DATABASE to the
  file in it. An existing volume can be moved there without losing
  data: its database.sqlite lands at exactly that path.
- Tested with a locally built image: a volume created by 2.0.1 with the
  old mount gets the migration once (not again on restart) and keeps
  its settings; the same volume moved to the new path keeps its data;
  a fresh volume with the new layout starts healthy.
- README and CHANGELOG (Unreleased) describe the new path and how to
  switch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -88,9 +88,13 @@ RUN rm -rf node_modules tests .gitea docker/dev.Dockerfile docker/dev-entrypoint
     bootstrap/cache/*.php \
     && mkdir -p storage/app/shares storage/app/public storage/framework/cache \
     storage/framework/sessions storage/framework/testing storage/framework/views \
-    storage/logs database \
+    storage/logs database/sqlite \
     && chmod -R 777 storage database bootstrap/cache
 
+# A docker-compose.yml from before 2.1.1 mounts the SQLite volume over all of database/, which hides
+# the migrations of every later image; the entrypoint adds the ones the volume is missing from here.
+RUN cp -R database/migrations docker/migrations
+
 # Create SQLite database file if it doesn't exist
 RUN touch database/database.sqlite \
     && chmod 666 database/database.sqlite