docker: Re-add ssh-client package

This has been removed from the image
by introducing --no-install-recommends.
It's dependency of libkas.py.

Dockerfile

@@ -13,7 +13,7 @@ RUN apt-get update && \
                        chrpath socat cpio python python3 rsync \
                        tar bzip2 curl dosfstools mtools parted \
                        syslinux tree python3-pip bc python3-yaml \
-                       lsb-release python3-setuptools && \
+                       lsb-release python3-setuptools ssh-client && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*