chmod start script from repo2docker-entrypoint

repo2docker/buildpacks/repo2docker-entrypoint

@@ -3,6 +3,7 @@
 # we use a login shell to get a fully configured shell environment
 # (e.g. sourcing /etc/profile.d, ~/.bashrc, and friends)
 if [[ ! -z "${R2D_ENTRYPOINT:-}" ]]; then
+    chmod u+x "$R2D_ENTRYPOINT"
     exec "$R2D_ENTRYPOINT" "$@"
 else
     exec "$@"