add busybox link to tar

See #2403

Author: drwetter

Dockerfile

@@ -26,7 +26,8 @@ FROM scratch
 ARG INSTALL_ROOT
 COPY --link --from=builder ${INSTALL_ROOT} /
 # Create user + (home with SGID set):
-RUN  echo 'testssl:x:1000:1000::/home/testssl:/bin/bash' >> /etc/passwd \
+RUN  ln -s /usr/bin/busybox /usr/bin/tar \
+  && echo 'testssl:x:1000:1000::/home/testssl:/bin/bash' >> /etc/passwd \
   && echo 'testssl:x:1000:' >> /etc/group \
   && echo 'testssl:!::0:::::' >> /etc/shadow \
   && install --mode 2755 --owner testssl --group testssl --directory /home/testssl \