docker-image: disable buildkit on armv7

buildkit is giving issues on armv7 after upgrading to musl-1.2. This is
most likely seccomp related. Without buildkit, there are no issues.

But we cannot blankly disable buildkit as the features we are using
require an up-to-date docker version, which is not the case everywhere
yet.

So disable this for armv7 so that we can continue building images.
1 job for master in 22 seconds
latest
Status Job ID Name Coverage
  Verify
passed #173314
docker-alpine
lint

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