inheritFrom requires specifying jar.manifest
https://github.com/johnrengelman/shadow/blob/5a80aa3aef898511d027d154a9732e55a5851d34/src/main/groovy/com/github/jengelman/gradle/plugins/shadow/ShadowJavaPlugin.groovy#L48

But the docs omit this: https://github.com/johnrengelman/shadow/blob/master/src/docs/asciidoc/10-configuring.adoc