Support multi-arch builds in dockerfile

[joshrosso]

built using:

docker buildx build --push -t joshrosso/secretgen-controller --platform linux/arm64,linux/amd64 .

Image available at:

docker.io/joshrosso/secretgen-controller:latest

manifest resulting in:

{
  "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
  "schemaVersion": 2,
  "manifests": [
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "digest": "sha256:118e3346384cdede1d09bea86afb85ec195cc036a1d13fc89d6369230f1b4ff2",
      "size": 1370,
      "platform": {
        "architecture": "arm64",
        "os": "linux"
      }
    },
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "digest": "sha256:cefcae98b52e1f09394cfcdf2370a727c7e33d4e652488f3d06460035e8f49f5",
      "size": 1370,
      "platform": {
        "architecture": "amd64",
        "os": "linux"
      }
    }
  ]
}

resolves: #57

Let me know if you'd like to take another approach.

[joe-kimmel-vmw]

Thanks @joshrosso - I know it's a little invisible but we've been talking about how best to support ARM builds here and in kapp-controller. @benmoss was exploring a different approach to organizing our checksums this morning. Appreciate your persistence and patience as we figure out how to balance our priorities.

[benmoss]

Similar comment to carvel-dev/kapp-controller#574 (comment), can you set defaults for TARGETOS/TARGETARCH so we can roll this out iteratively?

[cppforlife]

covered by #85