Ci workflows (#185)

* Build dev images with every commit.

* Use virtualenv to build dev images.

* Activate virtualenv properly.

.circleci/config.yml

@@ -58,6 +58,3 @@ workflows:
     jobs:
       - build:
           context: org-global
-          filters:
-            branches:
-              only: master

build.sh

@@ -12,7 +12,9 @@ fi
 REG_URL=${REGISTRY}_URL
 
 if [ "$EVENT_TYPE" = "cron" ]  || [ "$DEV" = "true" ] ; then
-		python -m pip install python-gitlab
+	python3 -m venv venv
+	source venv/bin/activate
+	pip install python-gitlab
 	# Get the latest commit for repositories and set their reference values to be
 	# used in the development builds.
 	CORE_REF=$(python get_latest_ref.py mailman/mailman)