appRoot property is not actually defaulted to /app

**Environment**:

- *Jib version:* `3.2.1`
- *Build tool:* Gradle
- *OS:* Mac OS Monterey 12.4, M1

**Description of the issue**:

The property reference `jib.container.appRoot` returns root container directory instead of default `/app`

It works properly when you have `container` section explicitly defined (see below) but it's not convenient as [the documentation says](https://github.com/GoogleContainerTools/jib/blob/master/jib-gradle-plugin/README.md#extended-usage) it should be `/app` by default

**Expected behavior**:

`jib.container.appRoot` always has the default value as it's documented

**Steps to reproduce**:

The next configuration:
```groovy
jib {
extraDirectories {
paths {
path {
from = "$project.rootDir/frontend/build"
into = "${jib.container.appRoot}/resources"
}
}
}
from {
image = "eclipse-temurin:17-jre"
}
}
```

Brings that inside the image files from `$project.rootDir/frontend/build` will be in `.` / root container directory and not the `/app` where other files are located

**Workaround**

Define container `container` **before** `extraDirectories`:

```groovy
jib {
container {
appRoot = "/app"
}
extraDirectories {
paths {
path {
from = "$project.rootDir/frontend/build"
into = "${jib.container.appRoot}/resources"
}
}
}
from {
image = "eclipse-temurin:17-jre"
}
}
```