dockerfile: fix escape parser directive

Signed-off-by: Hank Donnay <[email protected]>

Author: hdonnay

rhel/dockerfile/dockerfile.go

@@ -100,9 +100,9 @@ func (p *labelParser) Run() error {
 			if strings.Contains(v, `escape=`) {
 				eq := strings.IndexByte(v, '=')
 				if eq == -1 {
-					return fmt.Errorf("botched parser directive: %#q", i.val)
+					panic("string changed while parsing?")
 				}
-				esc, _ := utf8.DecodeRuneInString(v[:eq+1])
+				esc, _ := utf8.DecodeRuneInString(v[eq+1:])
 				p.lex.Escape(esc)
 				p.unquote.Escape(esc)
 				p.vars.Escape(esc)

rhel/dockerfile/testdata/Escape.txtar

@@ -0,0 +1,8 @@
+-- Dockerfile --
+# escape=ꙮ
+LABEL a ꙮ
+ꙮ$b
+-- Want --
+{
+  "a": "$b"
+}