fix(docker): exclude uploads/ from build context

The uploads/ directory is bind-mounted at runtime via docker-compose and
should not be copied into the Docker build context. This was slowing down
builds and including potentially large media files in the context.

.dockerignore

@@ -23,3 +23,6 @@ node_modules/
 
 # Environment
 .env
+
+# Media uploads (bind-mounted at runtime)
+uploads/