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

.gitignore

@@ -1,3 +1,5 @@
 /target
 *.db
 
+/staging/*
+!/staging/.gitkeep