rework: address review feedback on PR #126

Changes per sneak's review:
- Delete docker-compose.yml, add example stanza to README
- Define custom domain types: ImageID, ContainerID, UnparsedURL
- Use custom types in all function signatures throughout codebase
- Restore imageID parameter (as domain.ImageID) in deploy pipeline
- buildContainerOptions now takes ImageID directly instead of
  constructing image tag from deploymentID
- Fix pre-existing JS formatting (prettier)

make check passes with zero failures.

docker-compose.yml

@@ -1,18 +0,0 @@
-services:
-  upaas:
-    build: .
-    restart: unless-stopped
-    ports:
-      - "8080:8080"
-    volumes:
-      - /var/run/docker.sock:/var/run/docker.sock
-      - ${HOST_DATA_DIR:-./data}:/var/lib/upaas
-    environment:
-      - UPAAS_HOST_DATA_DIR=${HOST_DATA_DIR:-./data}
-      # Optional: uncomment to enable debug logging
-      # - DEBUG=true
-      # Optional: Sentry error reporting
-      # - SENTRY_DSN=https://...
-      # Optional: Prometheus metrics auth
-      # - METRICS_USERNAME=prometheus
-      # - METRICS_PASSWORD=secret