docker-image: use buildkit to set platform
We build armv7 and x86 images on a 64 bits host in 32 bits mode. Even though the images are 32 bits, docker still tags them as the host architecture. Docker has a way to specify the platform, but requries to enable experimental features on the daemon, but this is not ideal. An alternative is using buildkit, which has additional features for docker. This can be enabled by setting `DOCKER_BUILDKIT` to `1`. So lets use that so that we can specify `--platform` when running `docker build`.
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