Upgrade Dockerfile to leap 15.6

As EOL comes closer for openSUSE Leap 15.5 (https://en.opensuse.org/Lifetime)
an update is needed.

``busybox-util-linux`` and ``busybox-vi`` had to be removed as they don't exist
anymore. Busybox was added but hexdump was not provided by the vendor.
As busybox was compiled "properly" hexdump can be added by just linking to it.

This fixes #2563

Dockerfile

@@ -1,6 +1,6 @@
 # syntax=docker.io/docker/dockerfile:1
 
-ARG LEAP_VERSION=15.5
+ARG LEAP_VERSION=15.6
 ARG INSTALL_ROOT=/rootfs
 
 FROM opensuse/leap:${LEAP_VERSION} as builder
@@ -18,7 +18,8 @@ RUN source /etc/os-release \
   && zypper "${ZYPPER_OPTIONS[@]}" --gpg-auto-import-keys refresh \
   && rpm -e util-linux --nodeps \
   && zypper "${ZYPPER_OPTIONS[@]}" --non-interactive install --download-in-advance --no-recommends \
-       bash procps grep gawk sed coreutils busybox-util-linux busybox-vi ldns libidn2-0 socat openssl curl \
+       bash procps grep gawk sed coreutils busybox ldns libidn2-0 socat openssl curl \
+  && ln -s /usr/bin/busybox /usr/bin/hexdump \
   && zypper up -y \
   && zypper "${ZYPPER_OPTIONS[@]}" clean --all
 ## Cleanup (reclaim approx 13 MiB):