AppImage / Expose App Details in Dockerfile

### What is the problem or limitation you are having?

With the introduction of `dockerfile_extra_content`, users can now easily augment the Docker image creation process. To help users create more robust additions, the details of the user's app should be made available for de-referencing.

### Describe the solution you'd like

A straight forward approach is simply to add details of the App as build arguments to the `docker build` process similar to how `system_requires` from `pyproject.toml` is provided via `ARG SYSTEM_REQUIRES`.

For example, the name of app via `APP_NAME` or the path to the platform directory via `APP_PLATFORM_PATH`.

### Describe alternatives you've considered

Such information can be duplicated in to `dockerfile_extra_content`.

### Additional context

_No response_

The issue points to dockerfile_extra_content and the existing SYSTEM_REQUIRES build argument; locate the backend that assembles the docker build invocation and the AppImage platform configuration. Trace how app metadata is available there, then verify that the requested app details are exposed as build arguments and existing Docker builds remain working.

Written by the indexing model from the issue text.