feat: add container manager CLI commands and docker-compose overlay for admin container lifecycle

.env.example

@@ -9,6 +9,21 @@ SECRET_KEY=change-me
 # `make dev`). Set only to point at a different SQLite file.
 # DEVPLACE_DATABASE_URL=sqlite:////app/devplace.db
 
+# Persistent runtime data (zip archives, container workspaces). Lives OUTSIDE the
+# package and is never served via /static. Defaults to <repo>/var. The docker
+# daemon must be able to bind-mount this dir for container /app mounts; point it
+# at a persistent volume in production.
+# DEVPLACE_DATA_DIR=/var/lib/devplace
+
+# Container Manager (admin-only, enabled via docker-compose.containers.yml).
+# Host the /p/<slug> ingress proxy dials to reach a published container port.
+# On the host: 127.0.0.1 (default). Containerized app reaching host ports:
+# host.docker.internal.
+# DEVPLACE_CONTAINER_PROXY_HOST=host.docker.internal
+# GID of /var/run/docker.sock on the host (getent group docker | cut -d: -f3),
+# so the UID-1000 app can use the socket.
+# DOCKER_GID=999
+
 # Public origin for absolute URLs (SEO, canonical links, push). Empty = derive
 # from the request.
 DEVPLACE_SITE_URL=