Fix circleci config for synapse:latest docker upload

Give the image the right tag, so that we can push it.

.circleci/config.yml

@@ -13,8 +13,8 @@ jobs:
     machine: true
     steps:
       - checkout
-      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:${CIRCLE_SHA1} .
-      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:${CIRCLE_SHA1}-py3 --build-arg PYTHON_VERSION=3.6 .
+      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:latest .
+      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:latest-py3 --build-arg PYTHON_VERSION=3.6 .
       - run: docker login --username $DOCKER_HUB_USERNAME --password $DOCKER_HUB_PASSWORD
       - run: docker push matrixdotorg/synapse:latest
       - run: docker push matrixdotorg/synapse:latest-py3

changelog.d/4316.bugfix

+Fix synapse:latest docker upload