Add --compress flag to tar-split asm

The Go implementation of gzip is the only known to produce compressed
layers with the expected digest hashes.

This change allows compressed tar layer files to be produced, which is
useful for exporting layers from non-Go tools.

cmd/tar-split/asm.go

@@ -37,6 +37,12 @@ func CommandAsm(c *cli.Context) {
 		outputStream = fh
 	}
 
+	if c.Bool("compress") {
+		zipper := gzip.NewWriter(outputStream)
+		defer zipper.Close()
+		outputStream = zipper
+	}
+
 	// Get the tar metadata reader
 	mf, err := os.Open(c.String("input"))
 	if err != nil {

cmd/tar-split/main.go

@@ -69,6 +69,11 @@ func main() {
 					Value: "",
 					Usage: "relative path of extracted tar",
 				},
+				cli.BoolFlag{
+					Name:  "compress",
+					Usage: "gzip compress the output",
+					// defaults to false
+				},
 			},
 		},
 		{