make it pretty, grep `certbot-dns`

root/etc/s6-overlay/s6-rc.d/init-certbot-config/run

@@ -24,7 +24,7 @@ for i in "${SANED_VARS[@]}"; do
 done
 
 # Check for and install requested DNS plugins
-if echo ${DOCKER_MODS} | grep -q "universal-package-install" && echo ${INSTALL_PIP_PACKAGES} | grep -q "dns"; then
+if grep -q "universal-package-install" <<< "${DOCKER_MODS}" && grep -q "certbot-dns" <<< "${INSTALL_PIP_PACKAGES}"; then
     echo "**** Installing requested dns plugins ****"
     /etc/s6-overlay/s6-rc.d/init-mod-universal-package-install-add-package/run
     /etc/s6-overlay/s6-rc.d/init-mods-package-install/run