[Zero-install-bugs] copying source files before building with dup-src

The documentation for [0compile: Developers](
https://docs.0install.net/tools/0compile/developers/) instructs to add an
attribute to the implementation element, `compile:dup-src='../img/true'`. I
have tried to add this attribute to implementation and several other
elements. This results in an invalid XML "unbound prefix" error. How do I
add this attribute properly?

Attaching the feed I am working on as an example.