docker: Fix a few too many copies (3ffc238799).

docker: Additional tweaks.

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

docker/Dockerfile.ingredients

@@ -3,6 +3,9 @@
 FROM input AS source
 ARG git_checkout
 
+WORKDIR /
+COPY --link --from=input . .
+
 ADD --keep-git-dir . /usr/src/tuwunel
 WORKDIR /usr/src/tuwunel
 RUN <<EOF
@@ -22,9 +25,6 @@ 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 .