Parser errors on dockerfiles without instructions

See https://github.com/moby/moby/pull/38487 for more discussion.

Signed-off-by: Natasha Jarus <linuxmercedes@gmail.com>
(cherry picked from commit 2ec7d53b00)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

frontend/dockerfile/parser/parser.go

@@ -275,6 +275,11 @@ func Parse(rwc io.Reader) (*Result, error) {
 	if len(warnings) > 0 {
 		warnings = append(warnings, "[WARNING]: Empty continuation lines will become errors in a future release.")
 	}
+
+	if root.StartLine < 0 {
+		return nil, errors.New("file with no instructions.")
+	}
+
 	return &Result{
 		AST:         root,
 		Warnings:    warnings,

frontend/dockerfile/parser/testfiles-negative/only_comments/Dockerfile

@@ -0,0 +1,3 @@
+# Hello
+# These are just comments
+