Update multi-arch image tests

Ensure the test pull for all platforms uses a multi-arch image
Use the pause container for testing specific platforms
Update the image unpack test to be explicit about the platform to unpack

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

image_test.go

@@ -45,7 +45,7 @@ func TestImageIsUnpacked(t *testing.T) {
 	}
 
 	// By default pull does not unpack an image
-	image, err := client.Pull(ctx, imageName)
+	image, err := client.Pull(ctx, imageName, WithPlatform("linux/amd64"))
 	if err != nil {
 		t.Fatal(err)
 	}