Add Docker packaging for CodeGraphContext CLI

[Sakeeb91]

Summary

• add a Dockerfile and .dockerignore so the CLI can run without a local Python toolchain
• document building, running, and publishing the container image
• validate the image with a local docker build

Closes #391

Testing

• docker build -t codegraphcontext-test:latest .
• docker run --rm codegraphcontext-test:latest --help
• pytest # failed: server fixture requires NEO4J_* credentials

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