Improvement: Plugin/Docker: Enables image build for specified platform

[xiaolin-ninja]

Fixes #1881. Enables a platform HCL arg that is equivalent to docker build --platform flag. This only works with buildkit on (will fail with error).

Usage:

Add platform in waypoint.hcl

app "web" {
  build {
    use "docker" {
        platform = "linux/arm64/v8"
    }
  }

  deploy {
    use "docker" {
    }
  }
}

How to verify this works:

waypoint build
docker inspect <docker image> output should contain

"Architecture": "arm64",
        "Variant": "v8",
        "Os": "linux",

Merge after #1937.

Issue filed with Docker/Moby so that it may work without buildkit in the future.

🍐 with @izaaklauer thanks!

[hashicorp-cla]

All committers have signed the CLA.

[briancain]

Nice job @xiaolin-ninja ! I know this is still a draft, but it looks straightforward enough I figured I'd leave a review 😄 Looks good overall, just a few minor comments! ✨

[briancain]

I rebased this PR to fixup the merge conflict, fyi!