Add empty shops.json in Dockerfile for cloud mode

The MyAesCrypt.export_to_file method expects config/shops.json to
exist as a cache of AES keys for tenant lookups. This file is in
.gitignore (runtime-generated) but needs to exist with at least an
empty JSON structure on first boot.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -23,8 +23,9 @@ RUN bundle install --deployment --without development test --jobs 4
 # Copy application
 COPY . .
 
-# Create required directories
-RUN mkdir -p tmp/pids tmp/puma tmp/cache tmp/sockets log storage public/uploads
+# Create required directories and runtime files
+RUN mkdir -p tmp/pids tmp/puma tmp/cache tmp/sockets log storage public/uploads \
+    && echo '{"data":[]}' > config/shops.json
 
 # Precompile assets
 RUN RAILS_ENV=production SECRET_KEY_BASE=placeholder bundle exec rake assets:precompile 2>/dev/null || true