Fix: Create installDir and make property internal so it also works inside docker

Author: Siedlerchr

src/main/groovy/dev/jbang/gradle/tasks/JBangTask.groovy

@@ -77,7 +77,7 @@ class JBangTask extends DefaultTask {
     @Optional
     final ListProperty<String> trusts
     @InputDirectory
-    @Optional
+    @Internal
     final DirectoryProperty installDir
 
     @Inject