Upgrade Alpine Linux used in the Docker image (3.8 -> 3.10) (#5619)

Alpine Linux 3.8 is still supported, but it seems like
it's quite outdated now.

While Python should be the same on both, all other libraries, etc.,
are much newer in Alpine 3.9 and 3.10.

Signed-off-by: Slavi Pantaleev <slavi@devture.com>

changelog.d/5619.misc

+Base Docker image on a newer Alpine Linux version (3.8 -> 3.10)

docker/Dockerfile

@@ -16,7 +16,7 @@ ARG PYTHON_VERSION=3.7
 ###
 ### Stage 0: builder
 ###
-FROM docker.io/python:${PYTHON_VERSION}-alpine3.8 as builder
+FROM docker.io/python:${PYTHON_VERSION}-alpine3.10 as builder
 
 # install the OS build deps
 
@@ -55,7 +55,7 @@ RUN pip install --prefix="/install" --no-warn-script-location \
 ### Stage 1: runtime
 ###
 
-FROM docker.io/python:${PYTHON_VERSION}-alpine3.8
+FROM docker.io/python:${PYTHON_VERSION}-alpine3.10
 
 # xmlsec is required for saml support
 RUN apk add --no-cache --virtual .runtime_deps \