Add docker builder context

Dockerfile

@@ -4,6 +4,11 @@ ARG RENKU_BASE_IMAGE=renku/renkulab-py:3.10-0.25.0
 
 ########################################################
 #                Renku install section                 #
+FROM ${RENKU_BASE_IMAGE} as builder
+
+# 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=2.9.4
 
 # Install renku from pypi or from github if a dev version