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`.
1 job for master in 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Verify
passed #42804
docker-alpine
lint

00:00:35