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This is just a hunch, but did you try using `/` instead of `\\`? Forward-slashes work on Windows, too, and some tools expect them

I've changed the code as follows
```var apiImage = new Image("api-image", new()
{
   Build = new DockerBuildArgs
   {
      Context = "..",
      Dockerfile = "../myapp.api/Dockerfile",
      Platform = "linux/amd64",
   },
   ImageName = repo.RepositoryUrl,
   Registry = new RegistryArgs
   {
      Server = repo.RepositoryUrl,
      Username = username,
      Password = password
   }
});```
and got error
`error: error hashing dockerfile "../myapp.api/Dockerfile": could not open file ../myapp.api/Dockerfile: open ..\..\myapp.api\Dockerfile: The system cannot find the path specified.`

I’m also seeing a similar error on Windows. The same builder args work fine on MacOS.

What works for me on Windows:
```build: {
  context: imageDir,
  dockerfile: `${imageDir}\\Dockerfile`
}```

What _doesn't_ work for me on Windows:
```build: {
  context: imageDir
}```
or
```build: {
  context: imageDir,
  dockerfile: `${imageDir}/Dockerfile`
}```

okay, I figured it out. This one was a little weird.

I'm calling the function `buildImage` twice, in order to tag the image with both `latest` and a hash. When I call `buildImage` the 2nd time, fails with this error (where `imageDir` is `test-image`):

```error: error hashing dockerfile "test-image/Dockerfile": could not open file test-image/Dockerfile: open test-image\test-image\Dockerfile: The system cannot find the path specified.```

This *only* happens on Windows. I can fix it on windows by adding
```dockerfile:`${imageDir}\\Dockerfile````

but if I use `${_imageDir_}/Dockerfile` it doesn't work on Windows -- it works fine on Mac OS.

Confirmed that this is a regression in @pulumi/docker 4.x  from 3.x

Workaround is to convert `imageDir` to an absolute path

Filed a bug for this regression in @pulumi/docker: <https://github.com/pulumi/pulumi-docker/issues/578>