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>