Parameterize the Ubuntu version

.circleci/Dockerfile.ubuntu

@@ -1,4 +1,5 @@
-FROM ubuntu:16.04
+ARG TAG
+FROM ubuntu:${TAG}
 
 ENV WHEELHOUSE_PATH /tmp/wheelhouse
 ENV VIRTUALENV_PATH /tmp/venv

.circleci/Dockerfile.ubuntu-18.04

@@ -1,28 +0,0 @@
-FROM ubuntu:18.04
-
-ENV WHEELHOUSE_PATH /tmp/wheelhouse
-ENV VIRTUALENV_PATH /tmp/venv
-# This will get updated by the CircleCI checkout step.
-ENV BUILD_SRC_ROOT /tmp/project
-
-# language-pack-en included to support the en_US LANG setting.
-# iproute2 necessary for automatic address detection/assignment.
-RUN apt-get --quiet update && \
-    apt-get --quiet --yes install git && \
-    apt-get --quiet --yes install \
-        sudo \
-    	build-essential \
-	python2.7 \
-    	python2.7-dev \
-    	libffi-dev \
-    	libssl-dev \
-    	libyaml-dev \
-    	virtualenv \
-	language-pack-en \
-	iproute2
-
-# Get the project source.  Copying in the whole project source here isn't very
-# cache friendly but there's no Docker layer cache on CircleCI anyway!
-COPY . ${BUILD_SRC_ROOT}
-
-RUN "${BUILD_SRC_ROOT}"/.circleci/prepare-image.sh "${WHEELHOUSE_PATH}" "${VIRTUALENV_PATH}" "${BUILD_SRC_ROOT}"

.circleci/config.yml

@@ -378,8 +378,8 @@ jobs:
       - run:
           name: "Build Ubuntu images"
           command: |
-            docker build -t tahoelafsci/ubuntu:16.04 -f ~/project/.circleci/Dockerfile.ubuntu-16.04 ~/project/
-            docker build -t tahoelafsci/ubuntu:18.04 -f ~/project/.circleci/Dockerfile.ubuntu-18.04 ~/project/
+            docker build --build-arg TAG=16.04 -t tahoelafsci/ubuntu:16.04 -f ~/project/.circleci/Dockerfile.ubuntu ~/project/
+            docker build --build-arg TAG=18.04 -t tahoelafsci/ubuntu:18.04 -f ~/project/.circleci/Dockerfile.ubuntu ~/project/
       - run:
           name: "Push Ubuntu images"
           command: |