This removes some of the checks that were added in 0cba7740d4,
but should no longer be needed.
- `dockerfile.BuildFromConfig()` is used for `docker (container) commmit` and
`docker (image) import`. For `docker import`, we're failing early already.
For `commit`, it won't be possible to have a container that doesn't have the
right operating-system, so there's no need to validate.
- `dispatchRequest.getImageOrStage()`: simplify the check; all checks resulted
in an error on Windows, so it came down to "Windows does not support FROM scratch".
- `dispatchState.beginStage()`: `image.OperatingSystem()` already defaults to the
`runtime.GOOS` if unset, so remove the local default fallback.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
cfddecc3d2