test: use python:3-alpine as base for the web image

test/requirements/web/Dockerfile

@@ -1,6 +1,7 @@
 # Docker Image running one (or multiple) webservers listening on all given ports from WEB_PORTS environment variable
 
-FROM python:3
+FROM python:3-alpine
+RUN apk add --no-cache bash
 COPY ./webserver.py /
 COPY ./entrypoint.sh /
 WORKDIR /opt