Fix image pull after a failure

When resuming from a failed pull writer.Truncate() was not
seeking to the proper position in the file. This caused writes to
happen after the previously written content, instead of at the start
of the file.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

content/local/writer.go

@@ -2,6 +2,7 @@ package local
 
 import (
 	"context"
+	"io"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -167,5 +168,8 @@ func (w *writer) Truncate(size int64) error {
 	}
 	w.offset = 0
 	w.digester.Hash().Reset()
+	if _, err := w.fp.Seek(0, io.SeekStart); err != nil {
+		return err
+	}
 	return w.fp.Truncate(0)
 }