Make asciiDoctor gradle task generate reproducible output

sop-java-picocli/build.gradle

@@ -64,6 +64,7 @@ task generateManpageAsciiDoc(type: JavaExec) {
 
 apply plugin: 'org.asciidoctor.jvm.convert'
 asciidoctor {
+    attributes 'reproducible': ''
     dependsOn(generateManpageAsciiDoc)
     sourceDir = file("${project.buildDir}/generated-picocli-docs")
     outputDir = file("${project.buildDir}/docs")
@@ -71,4 +72,4 @@ asciidoctor {
     outputOptions {
         backends = ['manpage', 'html5']
     }
-}
+}