Linux / System / Add Support for Sourcing Build Requirements from Additional Package Repos

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

When targeting a RHEL derivative like Rocky Linux and AlmaLinux, they require a non-default package repo to be available to install `gobject-introspection-devel`.

To workaround this when its the host system:
```
sudo dnf --enablerepo=crb install gobject-introspection-devel
```

ref: https://github.com/beeware/briefcase/pull/1106#issuecomment-1457385059

### Describe the solution you'd like

Add the ability the enable arbitrary repos in `Dockerfile`.

### Describe alternatives you've considered

`Dockerfile` can be manually modified or a VM with the specific distro can be setup.

### Additional context

_No response_

Start by locating where the Dockerfile for Linux/RHEL-derived targets is generated or configured. Trace how package requirements are installed, then verify that arbitrary additional repositories can be enabled and that gobject-introspection-devel is installable on Rocky Linux or AlmaLinux.

Written by the indexing model from the issue text.