docker: Further optimize multi-layered build.

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

docker/Dockerfile.ingredients

@@ -22,6 +22,9 @@ ARG CARGO_HOME
 ARG CARGO_TARGET
 ARG CARGO_TERM_VERBOSE=0
 
+WORKDIR /
+COPY --link --from=input . .
+
 WORKDIR /usr/src/tuwunel
 COPY --link --from=source /usr/src/tuwunel .