ENH: Use local Debian repository first.

unittests/docker/Dockerfile

 FROM debian:latest
+# Use local package repository
+COPY sources.list.local /etc/apt/
+RUN mv /etc/apt/sources.list /etc/apt/sources.list.orig
+RUN cat /etc/apt/sources.list.local /etc/apt/sources.list.orig > /etc/apt/sources.list
 RUN apt-get update && \
     apt-get install -y \
       pylint3 python3-pip tox git \

unittests/docker/sources.list.local

+# Local repositories at Netcup
+deb http://debian.netcup.net/debian/ buster main
+deb http://mirrors.n-ix.net/debian-security buster/updates main
+deb http://debian.netcup.net/debian/ buster-updates main
+
+# The original content follows here:
\ No newline at end of file