Radicale: Use pip package instead of alpine repo

Required to fix failing builds caused by [alpine]upstream package rebuild against different python version

optional/radicale/Dockerfile

@@ -1,8 +1,8 @@
 ARG DISTRO=alpine:3.10
 FROM $DISTRO
 
-RUN echo "@testing http://nl.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories \
+RUN apk add --no-cache curl bash python3 \
- && apk add --no-cache radicale@testing curl bash
+ && pip3 install radicale
 
 COPY radicale.conf /radicale.conf
 

towncrier/newsfragments/1255.bugfix

@@ -0,0 +1 @@
+Use pip package for radicale to fix failing builds caused by [alpine]upstream package rebuild against different python version