Fix Docker build by adding SECRET_KEY_BASE to react_on_rails:locale task

The previous commit removed the ENV SECRET_KEY_BASE but the
react_on_rails:locale task still requires it to initialize Rails.

The SECRET_KEY_BASE is now set inline for both the locale task and
asset precompilation, ensuring it's only used during build and not
persisted in the final image.

This fixes the deployment error:
  ArgumentError: Missing `secret_key_base` for 'production' environment

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: justin808

.controlplane/Dockerfile

@@ -65,7 +65,8 @@ ENV RAILS_ENV=production \
     NODE_ENV=production
 
 # These files hardly ever change
-RUN bin/rails react_on_rails:locale
+# SECRET_KEY_BASE is required for Rails initialization but is not persisted in the image
+RUN SECRET_KEY_BASE=precompile_placeholder bin/rails react_on_rails:locale
 
 # These files change together, /app/lib/bs are temp build files for rescript,
 # and /app/client/app are the client assets that are bundled, so not needed once built