compression: support zstd with skippable frame

As a matter of fact, there are two frame formats defined by Zstandard: Zstandard frames and Skippable frames.
So we should probably support zstd algorithms with skippable frames.
See https://tools.ietf.org/id/draft-kucherawy-dispatch-zstd-00.html#rfc.section.2 for more details.

Signed-off-by: Da McGrady <dabkb@aol.com>

archive/compression/compression_test.go

@@ -189,3 +189,39 @@ func TestCmdStreamBad(t *testing.T) {
 		t.Fatalf("wrong output: %s", string(buf))
 	}
 }
+
+func TestDetectCompressionZstd(t *testing.T) {
+	for _, tc := range []struct {
+		source   []byte
+		expected Compression
+	}{
+		{
+			// test zstd compression without skippable frames.
+			source: []byte{
+				0x28, 0xb5, 0x2f, 0xfd, // magic number of Zstandard frame: 0xFD2FB528
+				0x04, 0x00, 0x31, 0x00, 0x00, // frame header
+				0x64, 0x6f, 0x63, 0x6b, 0x65, 0x72, // data block "docker"
+				0x16, 0x0e, 0x21, 0xc3, // content checksum
+			},
+			expected: Zstd,
+		},
+		{
+			// test zstd compression with skippable frames.
+			source: []byte{
+				0x50, 0x2a, 0x4d, 0x18, // magic number of skippable frame: 0x184D2A50 to 0x184D2A5F
+				0x04, 0x00, 0x00, 0x00, // frame size
+				0x5d, 0x00, 0x00, 0x00, // user data
+				0x28, 0xb5, 0x2f, 0xfd, // magic number of Zstandard frame: 0xFD2FB528
+				0x04, 0x00, 0x31, 0x00, 0x00, // frame header
+				0x64, 0x6f, 0x63, 0x6b, 0x65, 0x72, // data block "docker"
+				0x16, 0x0e, 0x21, 0xc3, // content checksum
+			},
+			expected: Zstd,
+		},
+	} {
+		compression := DetectCompression(tc.source)
+		if compression != tc.expected {
+			t.Fatalf("Unexpected compression %v, expected %v", compression, tc.expected)
+		}
+	}
+}