Update Dockerfile

Dockerfile

-FROM renku/renkulab-py:3.7-renku0.15.1-0.6.3
+ARG RENKU_BASE_IMAGE=renku/renkulab-py:3.7-0.7.2
+FROM ${RENKU_BASE_IMAGE}
+
+# FROM renku/renkulab-py:3.7-renku0.15.1-0.6.3
 # see https://github.com/SwissDataScienceCenter/renkulab-docker
 # to swap this image for the latest version available
 
+# RENKU_VERSION determines the version of the renku CLI
+# that will be used in this image. To find the latest version,
+# visit https://pypi.org/project/renku/#history.
+ARG RENKU_VERSION=0.15.1
+
 # Uncomment and adapt if code is to be included in the image
 # COPY src /code/src