Image resource on Windows without default platform results in invalid platform selection

[AaronFriel]

When building an image like so on Windows:

name: image-yaml
description: A Docker image build
runtime: yaml
config: {}
variables: {}
resources:
  demo-image:
    type: docker:Image
    properties:
      imageName: username/image:tag1
      skipPush: true
      build:
        context: ./app
    options:
      version: v4.0.0
outputs:
  imageName: ${demo-image.imageName}

With this Dockerfile:

FROM busybox:latest

COPY test.txt /

CMD ["cat", "test.txt"]

The program fails with:

  pulumi:pulumi:Stack: (same)
    [urn=urn:pulumi:dev::image-yaml::pulumi:pulumi:Stack::image-yaml-dev]
Building your image for windows\amd64 architecture.
To ensure you are building for the correct platform, consider explicitly setting the `platform` field on ImageBuildOptions.
    ~ docker:index/image:Image: (update)
        [id=username/image:tag1]
        [urn=urn:pulumi:dev::image-yaml::docker:index/image:Image::demo-image]
        [provider=urn:pulumi:dev::image-yaml::pulumi:providers:docker::default_4_0_0::50a92873-b9b3-40a7-b40a-f79f25bfcb1c]
      + build: {
          + context      : "./app"
          + contextDigest: "3ee3eaa9c20ce94aa69bd6d4752ad2cb5c114b1218aa326ac1b37d32f498cf0a"
          + platform     : "windows\\amd64"
        }
Building the image
error: Error response from daemon: "windows\\amd64" is an invalid component of "windows\\amd64": platform specifier component must match "^[A-Za-z0-9_-]+$": invalid argument
error: update failed

This appears to be the result of joining "windows" and "amd64" via filepath.Join().