Mirrors the verbose wait-for-docker block from build.yaml so future "failed
to connect to docker" failures print socket permissions, docker-related
env vars, and full `docker info` output instead of a single unhelpful line.
The previous approach (`docker run -v $(pwd)/rust/prop_grid_rs:/src`)
fails under act_runner because the runner container's pwd isn't a path
the host docker daemon can see — clippy installed, then cargo couldn't
find Cargo.toml.
Move the lint + test gate into the Dockerfile builder stage so the
build context ships over the socket the normal way. If clippy warns or
a test regresses, the image build fails and nothing is pushed.
Runs `cargo clippy --all-targets -- -D warnings` + `cargo test` inside
a rust:1.94-trixie container, then builds `rust/prop_grid_rs/Dockerfile`
and pushes three tags: the timestamped `main-<ts>-<sha>`, `latest`, and
a static `main` tag that `k8s/deployment-grid-rs.yaml` pins to.
Path-filtered to `rust/prop_grid_rs/**` so Elixir-only pushes don't
trigger a redundant Rust build.
