docker: Further optimize multi-layered build.

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

docker/Dockerfile.cookware

@@ -8,6 +8,9 @@ ARG CARGO_TARGET
 ARG rustup_version="1.28.1"
 ARG rustup_components
 
+WORKDIR /
+COPY --link --from=input . .
+
 WORKDIR /opt
 RUN \
 --mount=type=cache,dst=${RUSTUP_HOME},sharing=locked \
@@ -50,6 +53,9 @@ ARG CARGO_HOME
 ARG CARGO_TARGET
 ARG cargo_installs
 
+WORKDIR /
+COPY --link --from=input . .
+
 RUN \
 --mount=type=cache,dst=${RUSTUP_HOME},sharing=locked \
 --mount=type=cache,dst=${CARGO_HOME},sharing=locked \