Adjust dev images for caching and add workdir again

docker-files/debian.Dockerfile

@@ -37,12 +37,14 @@ RUN CASS_DRIVER_NO_CYTHON=1 pip3 install --no-cache-dir --user -r optional-requi
 
 FROM build as dev
 
-COPY . /glances
 COPY --from=additional-packages /root/.local/lib/python3.9/site-packages /usr/lib/python3.9/site-packages/
+COPY . /glances
 
 # EXPOSE PORT (XMLRPC / WebUI)
 EXPOSE 61209 61208
 
+WORKDIR /glances
+
 # Define default command.
 CMD python3 -m glances -C /glances/conf/glances.conf $GLANCES_OPT