net-rs/docker: add README covering build and deploy contract
PR #904 introduced the Dockerfile, entrypoint and relay.toml but
documented the build/run contract only in the Dockerfile's own header
comment. Adds a focused docker/README.md with:
- Build command (and the BuildKit / docker-buildx prerequisite).
- TOML config layering — mainnet.toml -> relay.toml -> env overlay
-> optional NET_NODE_CONFIG.
- Environment-variable table (port, peers, id-validation rule,
config path, RUST_LOG).
- Run examples for the common shapes: standalone relay, user overlay
via bind mount, non-default port.
- Container hygiene notes (non-root leios user, EXPOSE 3001,
signal handling, overlay write path).
net-rs/README.md gains a one-line pointer to the new doc next to the
existing "Building" section.
Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>