Upgrade to bullseye

Dockerfile

-FROM debian:buster
+FROM debian:bullseye-slim
 #
 # original author "Kirill Müller" <krlmlr+docker@mailbox.org>
 MAINTAINER "Michał 'rysiek' Woźniak" <rysiek@occrp.org>
@@ -47,13 +47,13 @@ RUN if [ "$INSTALL_PACKAGES" != "" ]; then \
         echo "* * * MAINLINE_PACKAGES: $MAINLINE_PACKAGES"; \
         BACKPORTS_PACKAGES="$( echo "$INSTALL_PACKAGES" | tr ' ' '\n' | egrep ':backports$' | sed -r -e 's/:backports$//' | tr '\n' ' ' )"; \
         echo "* * * BACKPORTS_PACKAGES: $BACKPORTS_PACKAGES"; \
-        [ "$BACKPORTS_PACKAGES" != "" ] && echo "deb http://ftp.debian.org/debian buster-backports main" > /etc/apt/sources.list.d/buster-backports.list ; \
+        [ "$BACKPORTS_PACKAGES" != "" ] && echo "deb http://ftp.debian.org/debian bullseye-backports main" > /etc/apt/sources.list.d/bullseye-backports.list ; \
         export DEBIAN_FRONTEND=noninteractive && apt-get update && apt-get install -y \
             $MAINLINE_PACKAGES \
             --no-install-recommends && \
         if [ "$BACKPORTS_PACKAGES" != "" ]; then \
             apt-get install -y \
-                -t buster-backports \
+                -t bullseye-backports \
                 $BACKPORTS_PACKAGES \
                 --no-install-recommends ; \
         fi; \