Log time to do fs sync on layer create (#349)

In our usage we are seeing occasional very large gaps between the final node in the build running and the subsequent layer commit. We suspect that fs sync before the layer diff is calculated is the cause and would like to get confirmation on this.

lib/snapshot/mem_fs.go

@@ -292,6 +292,8 @@ func (fs *MemFS) AddLayerByCopyOps(cs []*CopyOperation, w *tar.Writer) error {
 // It also waits at least one sec, in case mtime doesn't have sub-second
 // resolution.
 func (fs *MemFS) sync() {
+	start := time.Now()
+
 	// Ensure this function takes at least one sec.
 	block := make(chan interface{}, 1)
 	go func() {
@@ -303,6 +305,9 @@ func (fs *MemFS) sync() {
 	syscall.Sync()
 
 	<-block
+
+	duration := time.Since(start).Round(time.Millisecond)
+	log.Infow("* Synchronized file system", "duration", duration)
 }
 
 // createLayerByScan computes the differences between the file system and merged