docker: Reduce more copies.

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

docker/Dockerfile.ingredients

@@ -6,7 +6,7 @@ ARG git_checkout
 WORKDIR /
 COPY --link --from=input . .
 
-ADD --keep-git-dir . /usr/src/tuwunel
+ADD --link --keep-git-dir . /usr/src/tuwunel
 WORKDIR /usr/src/tuwunel
 RUN <<EOF
     set -eux
@@ -29,6 +29,9 @@ ARG JEMALLOC_OVERRIDE
 #ARG ZSTD_SYS_USE_PKG_CONFIG=0
 #ARG ROCKSDB_LIB_DIR
 
+WORKDIR /
+COPY --link --from=input . .
+
 WORKDIR /usr/src/tuwunel
 COPY --link --from=source /usr/src/tuwunel .
 
@@ -62,6 +65,9 @@ ARG RUSTUP_HOME
 ARG CARGO_HOME
 ARG CARGO_TARGET
 
+WORKDIR /
+COPY --link --from=input . .
+
 WORKDIR /usr/src/tuwunel
 RUN \
 --mount=type=cache,dst=${RUSTUP_HOME},sharing=locked \