apt: switch to a mirror closer to the build machine

Dockerfile

@@ -2,9 +2,14 @@
 # building the Reform system img file.
 
 FROM debian:bullseye-slim
+RUN sed -i 's|http://deb.|http://ftp.de.|g' /etc/apt/sources.list
 RUN apt -y update
-RUN apt-get -y install sudo gzip libext2fs2 pigz
-RUN apt-get -y install bc \
+RUN apt-get -y install 
+RUN apt-get -y install sudo \
+                       gzip \
+                       libext2fs2 \
+                       pigz \
+                       bc \
                        parted \
                        multistrap \
                        udisks2 \