Update docker image to use Python 3.7. (#5546)

Python 3.7 is apparently faster than 3.6, and should be mature enough.

.circleci/config.yml

@@ -4,7 +4,7 @@ jobs:
     machine: true
     steps:
       - checkout
-      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:${CIRCLE_TAG} -t matrixdotorg/synapse:${CIRCLE_TAG}-py3 --build-arg PYTHON_VERSION=3.6 .
+      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:${CIRCLE_TAG} -t matrixdotorg/synapse:${CIRCLE_TAG}-py3 .
       - run: docker login --username $DOCKER_HUB_USERNAME --password $DOCKER_HUB_PASSWORD
       - run: docker push matrixdotorg/synapse:${CIRCLE_TAG}
       - run: docker push matrixdotorg/synapse:${CIRCLE_TAG}-py3
@@ -12,7 +12,7 @@ jobs:
     machine: true
     steps:
       - checkout
-      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:latest -t matrixdotorg/synapse:latest-py3 --build-arg PYTHON_VERSION=3.6 .
+      - run: docker build -f docker/Dockerfile --label gitsha1=${CIRCLE_SHA1} -t matrixdotorg/synapse:latest -t matrixdotorg/synapse:latest-py3 .
       - 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/5546.feature

@@ -0,0 +1 @@
+Update docker image to use Python 3.7.

docker/Dockerfile

@@ -11,7 +11,7 @@
 #    docker build -f docker/Dockerfile --build-arg PYTHON_VERSION=3.6 .
 #
 
-ARG PYTHON_VERSION=2
+ARG PYTHON_VERSION=3.7
 
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 ### Stage 0: builder