Add Dockerfile, GitHub Action, and CI workflow

[baylesj]

Summary

• Dockerfile: Multi-stage build (rust:1-slim-bookworm → debian:bookworm-slim); only system dep is ca-certificates since reqwest uses rustls
• entrypoint.sh: Bash wrapper that maps INPUT_* env vars to CLI flags so the action can invoke the binary safely
• action.yml: Reusable GitHub Action pointing at docker://ghcr.io/baylesj/gerritoscopy:latest so users skip the compile step
• .github/workflows/ci.yml: cargo fmt --check + cargo test on every push/PR; build+push to GHCR on main or semver tags
• README.md: Full overhaul — description, prerequisites, copy-paste workflow snippet, inputs table, themes table, supported hosts table, local CLI section

Test plan

• cargo test passes locally (79/79)
• docker build -t gerritoscopy-test . completes without errors
• CI workflow triggers on this PR and all checks go green
• After merge to main, the docker job builds and pushes ghcr.io/baylesj/gerritoscopy:latest

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