Consider creating a `docker buildx build` test

[richlander]

We're updating the samples to be fully multi-platform friendly. It would be good to have a test that validates that, along the lines of the following. It would be most relevant if we could run it on both x64 and Arm64 machines.

docker buildx build --platform arm,arm64,amd64 -t foo .

Related to #4742

[lbussell]

In newer versions of Docker (including the one in alpine3.18 and our ImageBuilder image), docker build is aliased to docker buildx build [source]. I assume the intent of this issue is that we test the cross-platform build functionality of buildx against some of our samples.