docker: Fix target mount cache paths; relax locks on cargo home.

docker: Persist lychee cache.

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

docker/Dockerfile.cargo.audit

@@ -7,10 +7,14 @@ ARG RUSTUP_HOME
 ARG CARGO_HOME
 ARG audit_args=""
 
+WORKDIR /
+COPY --link --from=input . .
+
 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=${RUSTUP_HOME}/downloads,sharing=shared,ro \
+--mount=type=cache,dst=${CARGO_HOME}/registry,sharing=shared,ro \
+--mount=type=cache,dst=${CARGO_HOME}/git,sharing=shared,ro \
 <<EOF
     set -eux
     rustup run ${rust_toolchain} \