fix: add curl + HEALTHCHECK to Dockerfile

[chaodu-agent]

Summary

Add curl to the runtime image and define a HEALTHCHECK instruction in the Dockerfile.

Changes

• Install curl alongside ca-certificates in the runtime stage
• Add HEALTHCHECK directive: probes /healthz every 30s

Why

ghpool was frozen for 9 days without ECS detecting it — because no health check was configured and curl wasn't available in the image to support one.

With the HEALTHCHECK in the Dockerfile:

• Docker/ECS automatically monitors container health
• ECS task definitions inherit the Dockerfile health check if none is specified at the task level
• Frozen processes get detected and replaced within ~90s