Dockerfile: use COPY --link for source code as well

I missed the most important COPY in 637ca59375cdc26486e9451eea1aa6a9aac6154e

Copying the source code into the dev-container does not depend on the parent
layers, so can use the --link option as well.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit ff2342154b9c34925e0ddb7988b6925346d9efcb)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile

@@ -646,4 +646,4 @@ EOT
 # > make shell
 # > SYSTEMD=true make shell
 FROM dev-base AS dev
-COPY . .
+COPY --link . .