set ARG to be default at 2.7

Author: robkooper

Dockerfile

@@ -1,10 +1,10 @@
 FROM ubuntu:16.04
 
 # python version
-ARG PYTHON_VERSION=""
+ARG PYTHON_VERSION="2.7"
 
 # environment variables
-ENV PYTHON_VERSION=${PYTHON_VERSION:-"2.7"} \
+ENV PYTHON_VERSION=${PYTHON_VERSION} \
     RABBITMQ_URI="amqp://guest:guest@rabbitmq:5672/%2F" \
     RABBITMQ_EXCHANGE="clowder" \
     RABBITMQ_QUEUE="" \