Make sure start script is executable

Author: betatim

repo2docker/buildpacks/base.py

@@ -154,6 +154,7 @@
 
 # Add start script
 {% if start_script is not none -%}
+RUN chmod +x "{{ start_script }}"
 ENV R2D_ENTRYPOINT "{{ start_script }}"
 {% endif -%}