build: support more variations on context and dockerfile + iidfile

Signed-off-by: Tibor Vass <tibor@docker.com>

build/utils.go

@@ -0,0 +1,34 @@
+package build
+
+import (
+	"archive/tar"
+	"bytes"
+	"os"
+)
+
+// archiveHeaderSize is the number of bytes in an archive header
+const archiveHeaderSize = 512
+
+func isLocalDir(c string) bool {
+	st, err := os.Stat(c)
+	return err == nil && st.IsDir()
+}
+
+func isArchive(header []byte) bool {
+	for _, m := range [][]byte{
+		{0x42, 0x5A, 0x68},                   // bzip2
+		{0x1F, 0x8B, 0x08},                   // gzip
+		{0xFD, 0x37, 0x7A, 0x58, 0x5A, 0x00}, // xz
+	} {
+		if len(header) < len(m) {
+			continue
+		}
+		if bytes.Equal(m, header[:len(m)]) {
+			return true
+		}
+	}
+
+	r := tar.NewReader(bytes.NewBuffer(header))
+	_, err := r.Next()
+	return err == nil
+}