build python 3 docker images on circle CI (#3976)

.circleci/config.yml

@@ -4,18 +4,24 @@ jobs:
     machine: true
     steps:
       - checkout
-      - run: docker build -f docker/Dockerfile -t matrixdotorg/synapse:$CIRCLE_TAG .
+      - run: docker build -f docker/Dockerfile -t matrixdotorg/synapse:${CIRCLE_TAG} .
+      - run: docker build -f docker/Dockerfile -t matrixdotorg/synapse:${CIRCLE_TAG}-py3 --build-arg PYTHON_VERSION=3.6 .
       - run: docker login --username $DOCKER_HUB_USERNAME --password $DOCKER_HUB_PASSWORD
-      - run: docker push matrixdotorg/synapse:$CIRCLE_TAG
+      - run: docker push matrixdotorg/synapse:${CIRCLE_TAG}
+      - run: docker push matrixdotorg/synapse:${CIRCLE_TAG}-py3
   dockerhubuploadlatest:
     machine: true
     steps:
       - checkout
-      - run: docker build -f docker/Dockerfile -t matrixdotorg/synapse:$CIRCLE_SHA1 .
+      - run: docker build -f docker/Dockerfile -t matrixdotorg/synapse:${CIRCLE_SHA1} .
+      - run: docker build -f docker/Dockerfile -t matrixdotorg/synapse:${CIRCLE_SHA1}-py3 --build-arg PYTHON_VERSION=3.6 .
       - run: docker login --username $DOCKER_HUB_USERNAME --password $DOCKER_HUB_PASSWORD
-      - run: docker tag matrixdotorg/synapse:$CIRCLE_SHA1 matrixdotorg/synapse:latest
-      - run: docker push matrixdotorg/synapse:$CIRCLE_SHA1
+      - run: docker tag matrixdotorg/synapse:${CIRCLE_SHA1} matrixdotorg/synapse:latest
+      - run: docker tag matrixdotorg/synapse:${CIRCLE_SHA1}-py3 matrixdotorg/synapse:latest-py3
+      - run: docker push matrixdotorg/synapse:${CIRCLE_SHA1}
+      - run: docker push matrixdotorg/synapse:${CIRCLE_SHA1}-py3
       - run: docker push matrixdotorg/synapse:latest
+      - run: docker push matrixdotorg/synapse:latest-py3
   sytestpy2:
     machine: true
     steps:

changelog.d/3976.misc

+Build py3 docker images for docker hub too