image: Fix Windows usage of Image resource

The context dir must be validated and made into an absolute dir. Methods from
`docker/cli/image/build` are used to massage the input path and validate it
before using it in build options.

provider/image.go

@@ -15,6 +15,7 @@ import (
 	"github.com/moby/buildkit/session/auth/authprovider"
 	"github.com/moby/moby/registry"
 
+	clibuild "github.com/docker/cli/cli/command/image/build"
 	"github.com/docker/cli/cli/config"
 	"github.com/docker/cli/cli/config/configfile"
 	"github.com/docker/cli/cli/config/credentials"
@@ -106,6 +107,15 @@ func (p *dockerNativeProvider) dockerBuild(ctx context.Context,
 		return "", nil, fmt.Errorf("error reading ignore file: %w", err)
 	}
 
+	contextDir, err := clibuild.ResolveAndValidateContextPath(build.Context)
+	if err != nil {
+		return "", nil, fmt.Errorf("error resolving context: %w", err)
+	}
+
+	if err := clibuild.ValidateContextDirectory(contextDir, initialIgnorePatterns); err != nil {
+		return "", nil, fmt.Errorf("error validating context: %w", err)
+	}
+
 	// un-ignore build files so the docker daemon can use them
 	ignorePatterns := buildCmd.TrimBuildFilesFromExcludes(
 		initialIgnorePatterns,

provider/provider.go

@@ -132,7 +132,7 @@ func (p *dockerNativeProvider) Check(ctx context.Context, req *rpc.CheckRequest)
 		os = "linux"
 	}
 	arch := runtime.GOARCH
-	hostPlatform := filepath.Join(os, arch)
+	hostPlatform := os + "/" + arch
 	msg := fmt.Sprintf(
 		"Building your image for %s architecture.\n"+
 			"To ensure you are building for the correct platform, consider "+