Ensure shpy Docker image is pushed/pulled once every 6 months

Docker is introducing an image retention policy for free plans (of which the shpy organization on Docker Hub is) that will remove images which have not been pushed or pulled in 6 months. I agree with the policy of removing unused images, but would like shpy to hang around for developers who discover it to have easy access

I haven't checked if this is really an issue or not (I'm not sure how often our image is used), but worst case we can always auto-publish every few months. I think this makes sense, too, because we'll automatically be kept up to date with the latest versions of supported shells