build: add pull flag to force image pulling

This PR adds a flag to `docker build`. `docker build --pull .` will force the pulling of the image, even if it exists locally.
Force pulling the image will update the existing local image if there's one.

Fixes #4238
This is needed to make sure we don't change older API versions. This is similar to the checks for the `--rm` and `--force-rm` flags.