Add explanatory comments (#26177)

tools/dockerfile/distribtest/python_ubuntu1604_x64/Dockerfile

@@ -16,6 +16,10 @@ FROM ubuntu:16.04
 
 RUN apt-get update -y && apt-get install -y python python-pip
 
+# Necessary to fix virtualenv compatibility problems with python2.7
 RUN pip install --upgrade pip==19.3.1
 
+# Ubuntu's python-pip package installs pip to /usr/bin, whereas the upgraded
+# pip lives at /usr/local/bin/pip. We'll use the absolute path for now, since
+# this will all be replaced with python3.
 RUN /usr/local/bin/pip install virtualenv