Dockerize Spectogram

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Summary

• Add a multi-stage Dockerfile: builds the Go binary, then packages it with Python in a lightweight image for the existing two-step workflow
• docker-entrypoint.sh dispatches generate (runs the spectrogram CLI) and serve (runs python3 -m http.server) container commands
• Add a Docker Build job to CI to catch build breakage
• Document Docker usage in README

Test plan

• go build, go vet, gofmt, go test -race -cover pass locally
• CI Docker Build job builds the image successfully