asgen-config: add MediaBaseUrl for remote media to be downloadable

See merge request !3

Otherwise remote media like remote icons and screenshots are
only reference the original origins, having a much worse
data quality.

https://github.com/ximion/appstream-generator/blob/master/docs/asgen-config.md#description-of-fields

Multiple configuration fixes

See merge request !2

According to docs: "The 64x64 icon size must always be present and be cached.
 If this is not the case, appstream-generator will adjust the configuration
 internally and emit a warning."
That seems to be the case as I have seen the following error:

2021-12-19 11:01:19 - ERROR: The icon size 64x64 must always be present and be allowed to be cached. Configuration has been adjusted.

According to the documentation only 48, 64 and 128 are possible[1]
Additionally, running the generator with 256 as icon size produces
an error, failing to generate icons:

2021-12-19 10:57:07 - ERROR: Invalid icon size '256x256' selected in configuration, icon policy has been ignored.

[1] https://github.com/ximion/appstream-generator/blob/master/docs/asgen-config.md

Now that apk-polkit-rs is avaiable on more arches

This reverts commit d6d720d9.

Properly set the exportDir instead

Also add gen for aarch64, since gnome-software-plugin-apk is available
on aarch64 and x86_64

Apparently they previously didn't work because the named volume
pointed to an already exisiting dir, so its contents were copied into the
docker container