Revert "Require continuation char to be last char in a line"

This reverts commit 105bc63295,
which (although correct), resulted in a backward incompatible
change.

We can re-implement this in future, after this changes
goes through a deprecation cycle

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerfile/parser/parser.go

@@ -61,7 +61,7 @@ func SetEscapeToken(s string, d *Directive) error {
 		return fmt.Errorf("invalid ESCAPE '%s'. Must be ` or \\", s)
 	}
 	d.EscapeToken = rune(s[0])
-	d.LineContinuationRegex = regexp.MustCompile(`\` + s + `$`)
+	d.LineContinuationRegex = regexp.MustCompile(`\` + s + `[ \t]*$`)
 	return nil
 }