update single docker file to pull from mcr optionally. need second opinion on other ml-specific dockerfile
-FROM ubuntu:16.04
+# internal users should provide MCR registry to build via 'docker build . --build-arg REGISTRY="mcr.microsoft.com/mirror/docker/library/"'
+# public OSS users should simply leave this argument blank or ignore its presence entirely
+ARG REGISTRY=""
+FROM ${REGISTRY}ubuntu:16.04
ARG CONDA_VERSION=4.7.12
ARG PYTHON_VERSION=3.7
