test: verify Docker image builds and starts successfully

[icebear0828]

This PR adds a CI workflow named Docker Smoke Test to verify the Dockerfile and container runtime environment.
It runs on push to master and pull_request.

Key testing steps:

• Builds codex-proxy-test from the repo root Dockerfile.
• Runs the container with the default port 8080.
• Waits for up to 30s for the healthcheck to pass using docker inspect for the healthy status.
• Verifies the http://localhost:8080/health endpoint.
• Cleans up and repeats the test with a custom config/default.yaml mapping the port to 8090.
• Verifies the healthcheck and endpoint correctly reflect the new port 8090.

Fixes #120

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PR created automatically by Jules for task 2564536036965797434 started by @icebear0828

[google-labs-jules]

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[icebear0828]

Closing: overlaps with #136, and has stale package-lock.json + missing error handling.

[google-labs-jules]

Closing: overlaps with #136, and has stale package-lock.json + missing error handling.

Understood. Acknowledging that this work is now obsolete and stopping work on this task.