docker-for-java | Docker for Java Developers | Continuous Deployment library

I think you might be misinterpreting tutorial. I haven't seen the tutorial itself, but checking the documentation for the WildFly Docker image here there's a mention that you need to extend base image and add your war file inside:

To do this you just need to extend the jboss/wildfly image by creating a new one. Place your application inside the deployments/ directory with the ADD command (but make sure to include the trailing slash on the deployment folder path, more info). You can also do the changes to the configuration (if any) as additional steps (RUN command).

This means that you need to create a Dockerfile with approximately this contents (change your-awesome-app.war with the path to your war file):