Add zsh completion for 'docker build --add-host'

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -978,6 +978,7 @@ __docker_image_subcommand() {
         (build)
             _arguments $(__docker_arguments) \
                 $opts_help \
+                "($help)*--add-host=[Add a custom host-to-IP mapping]:host\:ip mapping: " \
                 "($help)*--build-arg=[Build-time variables]:<varname>=<value>: " \
                 "($help)*--cache-from=[Images to consider as cache sources]: :__docker_complete_repositories_with_tags" \
                 "($help -c --cpu-shares)"{-c=,--cpu-shares=}"[CPU shares (relative weight)]:CPU shares:(0 10 100 200 500 800 1000)" \