Correctly parse ONBUILD instructions

Fixes #23

buildkit/internal/dockerfile/dockerfile.go

@@ -85,25 +85,30 @@ func Print(node *parser.Node) (string, error) {
 }
 
 func printNode(node *parser.Node, writer io.Writer) error {
-	var v string
+	v := fmtNode(node)
 
+	_, err := fmt.Fprintln(writer, v)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func fmtNode(node *parser.Node) (v string) {
 	// format per directive
-	switch node.Value {
+	switch strings.ToLower(node.Value) {
 	// all the commands that use parseMaybeJSON
 	// https://github.com/moby/buildkit/blob/2ec7d53b00f24624cda0adfbdceed982623a93b3/frontend/dockerfile/parser/parser.go#L152
 	case command.Cmd, command.Entrypoint, command.Run, command.Shell:
 		v = fmtCmd(node)
 	case command.Label:
 		v = fmtLabel(node)
+	case command.Onbuild:
+		v = fmtSubCommand(node)
 	default:
 		v = fmtDefault(node)
 	}
-
-	_, err := fmt.Fprintln(writer, v)
-	if err != nil {
-		return err
-	}
-	return nil
+	return v
 }
 
 func getCmd(n *parser.Node) []string {
@@ -153,6 +158,16 @@ func fmtCmd(node *parser.Node) string {
 	return strings.Join(cmd, " ")
 }
 
+func fmtSubCommand(node *parser.Node) string {
+	cmd := []string{strings.ToUpper(node.Value)}
+	if len(node.Flags) > 0 {
+		cmd = append(cmd, node.Flags...)
+	}
+
+	sub := node.Next.Children[0]
+	return strings.Join(cmd, " ") + " " + fmtCmd(sub)
+}
+
 func fmtDefault(node *parser.Node) string {
 	cmd := getCmd(node)
 	return strings.Join(cmd, " ")

buildkit/internal/transform/transforms_test.go

@@ -120,3 +120,20 @@ FROM astro-runtime:7.0.0
 	preambleText, err := dockerfile.Print(preamble)
 	assert.Equal(t, expectedPreamble, preambleText)
 }
+
+func TestOnBuild(t *testing.T) {
+	testDockerfile := `# syntax=astronomer/astro-runtime
+FROM quay.io/astronomer/astro-runtime:7.0.0
+ONBUILD RUN echo "hi!"
+`
+
+	expectedDockerfile := `ONBUILD RUN echo "hi!"
+`
+
+	preamble, body, err := Transform([]byte(testDockerfile), nil)
+	require.NoError(t, err)
+	assert.NotNil(t, preamble)
+	assert.NotNil(t, body)
+	bodyText, err := dockerfile.Print(body)
+	assert.Equal(t, expectedDockerfile, bodyText)
+}