use tty cmd to detect if we have one. closes #921

plugins/00_dokku-standard/commands

@@ -49,8 +49,8 @@ case "$1" in
     shift 2
 
     DOCKER_ARGS=$(: | pluginhook docker-args $APP run)
-    # if we have a prompt, then we have a tty according to the LDP (http://www.tldp.org/LDP/abs/html/intandnonint.html)
-    [[ ! -z "$PS1" ]] && DOKKU_RUN_OPTS="-i -t"
+    [[ "$(/usr/bin/tty || true)" != "not a tty" ]] && DOKKU_RUN_OPTS="-i -t"
+
     docker run $DOKKU_RUN_OPTS $DOCKER_ARGS $IMAGE /exec "$@"
     ;;