Download the appropriate package archive for your Kobo model. Some might work on others, but we can't guarantee that. These are squashfs XZ-compressed files, mountable like that:
mount $FILE $MOUNTPOINT. If you have Windows, you can extract it with 7-Zip. If your Kobo is not listed here, then it is likely natively supported by the latest Alpine Linux version.
Kobo Mini/Touch (2021-02-10, hopefully)