Add custom port testing to Docker CI pipeline

[icebear0828]

This PR updates the GitHub Actions docker smoke test to run as a matrix job against multiple port configurations (the default 8080 and custom 8090). It ensures the container starts reliably via the configured PORT env var and correctly validates its /health status endpoints over the requested port.

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PR created automatically by Jules for task 6109901273709554691 started by @icebear0828

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