Simplify Quick Start: single docker compose up

- Dockerfile: auto-generate DUMPSTER_SECRET if not set
- compose.yml: remove env_file requirement, use env vars with defaults, share web_assets volume with caddy
- README: Quick Start is now 3 commands (clone, cd, docker compose up)

docker/Dockerfile

@@ -19,4 +19,4 @@ WORKDIR /app
 COPY --from=go-builder /bin/dumpster-server /app/dumpster-server
 COPY --from=web-builder /app/dist /srv/web
 EXPOSE 8080
-CMD ["/app/dumpster-server"]
+CMD ["sh", "-c", "if [ -z \"$DUMPSTER_SECRET\" ]; then export DUMPSTER_SECRET=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | head -c 32); fi; /app/dumpster-server"]