Match Mount default cache/sync to container defaults. (#446)

- Use `cache=auto`, `sync=fsync` as default everywhere.

Author: jglogan

Sources/Containerization/Mount.swift

@@ -164,7 +164,7 @@ extension Mount {
 extension VZDiskImageStorageDeviceAttachment {
     static func mountToVZAttachment(mount: Mount, options: [String]) throws -> VZDiskImageStorageDeviceAttachment {
         var cachingMode: VZDiskImageCachingMode = .automatic
-        var synchronizationMode: VZDiskImageSynchronizationMode = .none
+        var synchronizationMode: VZDiskImageSynchronizationMode = .fsync
 
         for option in options {
             let split = option.split(separator: "=")