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

README.md

@@ -2,3 +2,28 @@
 
 My super simple CDN built for distributing my own (text) content. 
 
+## Running in Docker
+
+```
+docker compose up -d
+```
+
+The runtime image is `distroless` (no shell, no package manager, ~24MB),
+so managing packages happens via `docker exec` running the `ccc` binary
+directly rather than an interactive shell:
+
+```
+docker compose exec ccc /usr/local/bin/ccc create demo
+```
+
+To add a file, it needs to exist inside the container first. Drop it in
+the `./staging` dir, which is bind-mounted to `/staging`:
+
+```
+cp app.js staging/
+docker compose exec ccc /usr/local/bin/ccc add demo /staging/app.js 1.0.0
+```
+
+(Alternatively `docker cp` a file straight into the container if you'd
+rather not use the staging mount.)
+