ensure correct permissions (enforced)

increases robustness to permissions in root image

.build/Dockerfile

@@ -27,7 +27,7 @@ ARG NB_GID="100"
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
 USER root
-
+RUN chmod 1777 /tmp && chmod 1777 /var/tmp
 # Install all OS dependencies for notebook server that starts but lacks all
 # features (e.g., download as all possible file formats)
 ENV DEBIAN_FRONTEND noninteractive