build-push-to-dockerhub caches by default

[mem]

The action sets cache-to to "type=gha,mode=max" by default, and this is trashing the cache, because every single PR and every single update in a PR is adding stuff to the cache.

1. This behavior should be documented, so that it doesn't catch users by surprise
2. How to disable this should be documented (I assume setting cache-to to an empty value should be enough, I have not tried)

The cache for my repo is full of buildkit-blob-1-sha256:.... This is problematic because GitHub limits the size of the cache, and it starts evicting older entries as the cache becomes full. This has the potential to evict useful entries (I'm not sure if GitHub takes last access into account for eviction, or if it only looks at creation time; I would hope it's the former, but I do not know). At any rate, the large number of entries (I have ~ 200) makes it difficult to work with the list, which hinders debugging.