Update runtime docker image to Alpine v3.11

changelog.d/7398.docker

+Update docker runtime image to Alpine v3.11. Contributed by @Starbix.

docker/Dockerfile

@@ -55,7 +55,7 @@ RUN pip install --prefix="/install" --no-warn-script-location \
 ### Stage 1: runtime
 ###
 
-FROM docker.io/python:${PYTHON_VERSION}-alpine3.10
+FROM docker.io/python:${PYTHON_VERSION}-alpine3.11
 
 # xmlsec is required for saml support
 RUN apk add --no-cache --virtual .runtime_deps \