Build randomly use old docker image & fail

I have a circleci 2.0 build, on a private project.
I use a custom image with a few tools:
https://hub.docker.com/r/lsenta/gcloud-functions-testing/

I’m seeing a lot of instances where my build will fail because a command is missing (lein for clojure(script)). Rebuilding, once or twice works.

It’s as if the circleci builder is not using the last version of my image every time.

Btw, even though this specific bug is annoying,
Thanks for providing this nice tool for free for open source and starters projects!