added 'docker build --taget' to zsh completion

Signed-off-by: Miroslav Gula <miroslav.gula@naytrolabs.com>

contrib/completion/zsh/_docker

@@ -1019,6 +1019,7 @@ __docker_image_subcommand() {
                 "($help)*--shm-size=[Size of '/dev/shm' (format is '<number><unit>')]:shm size: " \
                 "($help)--squash[Squash newly built layers into a single new layer]" \
                 "($help -t --tag)*"{-t=,--tag=}"[Repository, name and tag for the image]: :__docker_complete_repositories_with_tags" \
+                "($help)--target=[Set the target build stage to build.]" \
                 "($help)*--ulimit=[ulimit options]:ulimit: " \
                 "($help)--userns=[Container user namespace]:user namespace:(host)" \
                 "($help -):path or URL:_directories" && ret=0