Only chmod u+x if not already executable, in case user is not owner but file is already g/o+x

repo2docker/buildpacks/repo2docker-entrypoint

@@ -3,7 +3,9 @@
 # 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"
+    if [[ ! -x "$R2D_ENTRYPOINT" ]]; then
+        chmod u+x "$R2D_ENTRYPOINT"
+    fi
     exec "$R2D_ENTRYPOINT" "$@"
 else
     exec "$@"