client: lib_docker: make fatal if user not given

client/src/docker/lib/lib_docker.bash

@@ -78,6 +78,7 @@ function lib_docker::docker()
   IFS="/" read -ra _image <<<"$1"
   declare -gx global_platform="${_image[0]}"
   local -r _user="${_image[1]}"
+  [ -z "$_user" ] && lib_utils::die_fatal "User not given for '${_image[*]}'"
   lib_utils::print_debug "global_platform=${global_platform}"
 
   if [[ ! "$global_platform" =~ ^archlinux$|^ubuntu$|^dev-tools$ ]]; then