Classic builder cache poisoning

The classic builder cache system is prone to cache poisoning if the image is built `FROM scratch`. Also, changes to some instructions (most important being `HEALTHCHECK` and `ONBUILD`) would not cause a cache miss.

An attacker with the knowledge of the Dockerfile someone is using could poison their cache by making them pull a specially crafted image that would be considered as a valid cache candidate for some build steps.

For example, an attacker could create an image that is considered as a valid cache candidate for: ``` FROM scratch MAINTAINER Pawel ```

when in fact the malicious image used as a cache would be an image built from a different Dockerfile.

In the second case, the attacker could for example substitute a different `HEALTCHECK` command.

### Impact

23.0+ users are only affected if they explicitly opted out of Buildkit (`DOCKER_BUILDKIT=0` environment variable) or are using the `/build` API endpoint (which uses the classic builder by default).

All users on versions older than 23.0 could be impacted. An example could be a CI with a shared cache, or just a regular Docker user pulling a malicious image due to misspelling/typosquatting.

Image build API endpoint (`/build`) and `ImageBuild` function from `github.com/docker/docker/client` is also affected as it the uses classic builder by default.

### Patches

Patches are included in Moby releases:

- v25.0.2 - v24.0.9 - v23.0.10

### Workarounds

- Use `--no-cache` or use Buildkit if possible (`DOCKER_BUILDKIT=1`, it's default on 23.0+ assuming that the buildx plugin is installed). - Use `Version = types.BuilderBuildKit` or `NoCache = true` in `ImageBuildOptions` for `ImageBuild` call.