Add staging volume and docker exec workflow docs

distroless has no shell, so managing packages against a running
container means exec-ing the ccc binary directly. Files being added
need to exist inside the container already, so bind-mount a host
./staging dir to /staging for that.

- docker-compose.yml: mount ./staging:/staging alongside the data volume
- README: document docker compose exec + staging workflow

docker-compose.yml

@@ -5,6 +5,9 @@ services:
       - "8333:8333"
     volumes:
       - ccc-data:/data
+      # Bind-mount a host dir here to stage files for `ccc add`, e.g.:
+      #   docker compose exec ccc /usr/local/bin/ccc add demo /staging/app.js 1.0.0
+      - ./staging:/staging
     restart: unless-stopped
 
 volumes: