feat(relay): add Dockerfile (#1661)

This adds a basic Dockerfile for the relay so users and devs can easily
start it.

rust/.dockerignore

@@ -0,0 +1 @@
+target/

rust/relay.Dockerfile

@@ -0,0 +1,23 @@
+# syntax=docker/dockerfile:1.5-labs
+FROM rust:1.70-slim as builder
+
+WORKDIR /workspace
+ADD . .
+RUN --mount=type=cache,target=./target \
+    --mount=type=cache,target=/usr/local/cargo/registry \
+    --mount=type=cache,target=/usr/local/rustup \
+    rustup target add x86_64-unknown-linux-musl && \
+    cargo build --release --bin relay --target x86_64-unknown-linux-musl
+
+RUN --mount=type=cache,target=./target \
+    mv ./target/x86_64-unknown-linux-musl/release/relay /usr/local/bin/relay
+
+FROM scratch
+COPY --from=builder /usr/local/bin/relay /usr/local/bin/relay
+ENV RUST_BACKTRACE=1
+
+EXPOSE 3478/udp
+EXPOSE 49152-65535/udp
+
+# This purposely does not include an `init` process. Use `docker run --init` for proper signal handling.
+ENTRYPOINT ["relay"]

rust/relay/README.md

@@ -22,6 +22,11 @@ You can build the server using: `cargo build --release --bin relay`
 
 For a detailed help text and available configuration options, run `cargo run --bin relay -- --help`.
 
+## Docker
+
+There is a docker image one directory up from this README: [Dockerfile](../relay.Dockerfile).
+The Rust binary itself does not include any signal handling, thus you need to run the container with `--init` if you want to be able to CTRL+C the running container.
+
 ## Design
 
 The relay is designed in a sans-IO fashion, meaning the core components do not cause side effects but operate as pure, synchronous state machines.