test: verify Docker image builds and starts successfully

[icebear0828]

This PR resolves issue #120 by introducing a matrix strategy to .github/workflows/ci-docker.yml to test the Docker container on both the default port (8080) and a custom overridden port (8090).

Changes:

• Updated ci-docker.yml with a strategy.matrix configuration [8080, 8090].
• Mapped docker run to the matrix port via -p and injected the -e PORT environment variable.
• Generated a local.yaml file on the fly with the port configuration to mount as a volume, effectively testing the configuration override loading flow.
• Refactored docker-healthcheck.sh to correctly resolve the listening port from the PORT env variable, the local.yaml overlay, or the default config, ensuring the internal healthcheck passes when the container is run on non-default ports.

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PR created automatically by Jules for task 17754268599624577203 started by @icebear0828

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