Added instructions on how to use Dockerfile

Author: ericmjl

utils/Dockerfile

@@ -1,3 +1,11 @@
+##### INSTRUCTIONS TO USE #####
+# Move this to the base directory of the repo
+# before using Docker to build the container.
+# This file was moved into `utils/`
+# because it was overriding the original
+# conda environment-based build using `environment.yml`.
+##### END INSTRUCTIONS #####
+
 # Base image: miniconda3
 FROM continuumio/miniconda3