feat: provide an OCI container image

[yesyayen]

What

Provide a first-class OCI container image for ExtendDB and a Docker Compose demo that brings up PostgreSQL, runs extenddb init, and starts the server with a working /health endpoint. Ships a small bootstrap helper so the "first DynamoDB request" path is just three command.

Demo

Why

Closes #56

Lower the friction for trying out ExtendDB. Today there is no container image; setup needs Rust, PostgreSQL, and several manual steps. With this PR, docker compose up && ./bootstrap-iam.sh produces a working DynamoDB endpoint in under a minute, which should help adoption for evaluation, demos, and CI.

Testing done

End-to-end on linux/amd64: build, init, serve, health, SigV4 round-trip via aws CLI, SIGTERM and SIGINT shutdown, idempotent restart. The GIF above shows the clone-to-list-tables flow.

No Rust source modifications, so cargo fmt, cargo clippy, and cargo test --workspace are not affected.

Explicit follow-ups (not in this PR)

1. GitHub Actions workflow that builds and publishes the image to GHCR.
2. Foreground / --no-detach mode for extenddb serve, which collapses docker/entrypoint.sh to exec extenddb "$@".

Checklist

• I have read CONTRIBUTING.md
• All tests pass (cargo test --workspace)
• Code is formatted (cargo fmt --check)
• Clippy is clean (cargo clippy -- -W clippy::pedantic)
• [N/A] I have added or updated tests for new functionality
• I have updated documentation if behavior changed
• Breaking changes are noted below (if any)

Breaking changes

None. This is purely additive.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache License 2.0 and I agree to the Developer Certificate of Origin (DCO). See CONTRIBUTING.md for details.