[ci skip] ARG does not survive past FROM

Hardcode the tag in two places

Author: parente

binder/Dockerfile

@@ -1,6 +1,5 @@
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
-ARG TAG=17aba6048f44
-FROM jupyter/base-notebook:$TAG
-ENV TAG=$TAG
+FROM jupyter/base-notebook:17aba6048f44
+ENV TAG="17aba6048f44"
 COPY binder/README.ipynb .