“Punk rockers” was definitely a thing. I read “punker” as a shortening of that.

I've only ever heard it at the end of that one Vandals song about rednecks: "I couldn't make it as a punker."

In the 1990 Teenage Mutant Ninja Turtles movie Casey Jones asks Raphael if he's some sort of punker, and then states that he hates punkers.

I watched that movie WAY too much as a kid.

In German it’s common to call Punks Punker.