Jlink is unable to derive module descriptor?

Question

I have a hello-world application that I have compiled into a .jar.

public class Main {
  public static void main(final String[] args) {
    System.out.println("Hello, world. ");
  }
}

Now I would like to use JLink to create a self-contained executable:

$ jlink --output testout --module-path ./out
Error: Unable to derive module descriptor for ./out/hello-app.jar

• What does this error mean?
• How can I fix this?

---

$ javac -version
javac 10.0.1

$ jlink --version
10.0.1

I am on Ubuntu 18.04.

---

Update:

I have added a --add-modules flag. Now my command is:

jlink --module-path $(location :hello-app) --add-modules hello --output $OUT

I have added a module-info.java file. My source tree:

$ tree src/
src/
├── hello
│   └── Main.java
└── module-info.java

And module-info.java:

module hello {

}

However, now the error message is:

Error: Module hello not found