Update Dockerfile

Dockerfile

@@ -9,4 +9,5 @@ RUN pip install -r /tmp/requirements.txt
 
 RUN $CONDA_DIR/envs/renku/bin/pip install -U -e git+https://github.com/SwissDataScienceCenter/renku-python.git#egg=renku
 
+RUN sudo apt-get update
 RUN sudo apt-get install -y poppler-utils