Haven't seen that error yet, but I'd suggest trying other Alpine mirrors (set them in your /etc/apk/repositories): https://mirrors.alpinelinux.org/
(there's also "apk fix" which helps when installing a package failed earlier for whatever reason, but that doesn't seem to be the cause here)
The bug gets tracked in https://gitlab.com/postmarketOS/pmaports/-/issues/2299, and a fix is being developed. Workarounds are mentioned in that issue. The download page has been updated to mention this and link to the issue in the meantime.
Is Alpine also becoming immutable, or is this development towards immutable independent/separate from Alpine?
It's a separate effort. 🙂
From the features table, looks like it barely boots up with the downstream kernel and can use the display. Check it out if you are curious and want to extend the port, or if you can come up with your own use case.
For anyone wondering why this has a thumbnail, and the other episode does not:
https://gitlab.com/postmarketOS/cast.postmarketos.org/-/merge_requests/5
I'm surprised how many sites use that tag, this one line change surely was worth it :D
It isn't really convenient to do that at the moment, but here is a guide that involves some manual looking around in pmaports: https://wiki.postmarketos.org/wiki/Configure_postmarketOS_for_multiple_UIs_or_users