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>

docker/entrypoint.sh

@@ -22,6 +22,16 @@ max_input_time = ${PHP_MAX_INPUT_TIME:-300}
 memory_limit = ${PHP_MEMORY_LIMIT:-512M}
 EOF
 
+# A docker-compose.yml from before 2.1.1 mounts the SQLite volume over all of /app/database, so the
+# migrations folder is the one the volume was created with: add this image's newer migrations to it.
+for migration in docker/migrations/*.php; do
+    if [ ! -e "database/migrations/${migration##*/}" ]; then
+        echo "[entrypoint] Adding migration ${migration##*/} to the database volume..."
+        mkdir -p database/migrations
+        cp "$migration" database/migrations/
+    fi
+done
+
 echo "[entrypoint] Running database migrations..."
 php artisan migrate --force