Support image's own entrypoint and published ports

[achimnol]

As a follow-up to #2582 (#1684 and #758), let's support the container image's own entrypoint scripts and published ports natively.

• Image's own entrypoint
  • Often they execute a service process (e.g., redis/postgres containers) or start other background processes as needed.
  • Since we override the container entrypoint with entrypoint.sh, our entrypoint script should invoke them as a background task.
• Image's own published ports
  • Since there is no service-port declaration, we could just assume that those are predefined "pre-open" service ports.
  • Pre-open service ports do not have explicit initialization steps managed by Backend.AI kernel runner, but the manager just notifies and registers the TCP ports of the container to the App Proxy for external access.