New scale-app command: scaling a container to N instances

v0.19.0-beta

New scale-app command: scaling a container to N instances. Until now a container could only be stopped or started (1 instance). The agent can now receive an arbitrary instance target from the Hub and runs docker compose up --scale <service>=<n> against the project's already-deployed compose file, without a re-render or a full redeployment. Backwards compatible: an older agent receiving this command would simply answer "unknown command type" without crashing.