Add package workflow.

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

docker/Dockerfile.cargo

@@ -20,9 +20,8 @@ WORKDIR /usr/src/tuwunel
 RUN \
 --mount=type=cache,dst=${RUSTUP_HOME},sharing=locked \
 --mount=type=cache,dst=${CARGO_HOME},sharing=locked \
---mount=type=cache,dst=${CARGO_TARGET_DIR},sharing=shared \
+--mount=type=cache,dst=${CARGO_TARGET_DIR},sharing=locked \
 <<EOF
-    env
     set -eux
     rustup run ${rust_toolchain} \
         cargo ${cargo_cmd} \