Add rustfmt check to workflow.

Signed-off-by: Jason Volk <jason@zemos.net>

docker/Dockerfile.cargo.fmt

@@ -0,0 +1,23 @@
+# syntax = docker/dockerfile:1.11-labs
+
+FROM input AS cargo
+ARG sys_target
+ARG rust_toolchain="nightly"
+ARG RUSTUP_HOME
+ARG CARGO_HOME
+ARG fmt_args=""
+
+WORKDIR /usr/src/tuwunel
+RUN \
+--mount=type=cache,dst=${RUSTUP_HOME},sharing=locked \
+--mount=type=cache,dst=${CARGO_HOME},sharing=locked \
+<<EOF
+    env
+    set -eux
+    rustup run ${rust_toolchain} \
+        cargo fmt \
+            --all \
+            --check \
+            --manifest-path Cargo.toml \
+            ${fmt_args}
+EOF