Using "Force Pull" never pulls from the registry

I'm using Docker Build and Publish to build an image using a non-default Dockerfile and publish to the repository. The build and publish steps work fine, but I don't want to go through all the build steps every time - I want new builds to take all non-changed steps from the Docker cache.

This works fine if I issue sequential builds on the same slave, but as I'm using on-demand EC2 slaves, these go down and when I start a new build it has to run through all the steps on the new slave.

I was under the impression that the "Force Pull" checkbox under "Advanced" (documented as "Update the source image before building even when it exists locally") would issue a `docker pull` from the registry before starting the build (though from the text, it looks like it should have pulled on a new slave anyway?) - but that doesn't seem to work: no pull is shown in the log, and the build is always started from scratch.

My configuration is shown in the screenshot