fix: Raise error when a created docker container exposes no port (#1645)

Backported-from: main
Backported-to: 23.03

changes/1645.fix.md

@@ -0,0 +1 @@
+Handle `None` value of newly created Docker container's port.

src/ai/backend/agent/docker/agent.py

@@ -913,7 +913,10 @@ async def start_container(
                 if container_config["HostConfig"].get("NetworkMode") == "host":
                     host_port = host_ports[idx]
                 else:
-                    host_port = int((await container.port(port))[0]["HostPort"])
+                    ports: list[dict[str, Any]] | None = await container.port(port)
+                    if ports is None:
+                        raise ContainerCreationError(container_id=cid)
+                    host_port = int(ports[0]["HostPort"])
                     assert host_port == host_ports[idx]
                 if port == 2000:  # intrinsic
                     repl_in_port = host_port