split  post build script command in Docker file
`RUN chmod +x postBuild && ./postBuild` causes `./postBuild: Text file busy` error with Docker version 1.13.1
@@ -109,7 +109,8 @@ USER ${NB_USER}
# Make sure that postBuild scripts are marked executable before executing them
{% if post_build_scripts -%}
{% for s in post_build_scripts -%}
-RUN chmod +x {{ s }} && ./{{ s }}
+RUN chmod +x {{ s }}
+RUN ./{{ s }}
{% endfor %}
{% endif -%}
