External libraries are no longer readonly in the latest release as many users wanted to be able to delete photos in external libraries, but you can mount the Docker volume containing your external library as readonly if the readonly aspect is important to you.
Yeah, saw that note earlier in the week. I have it set to read-only through docker for now, but I like the idea of being able to edit metadata. We'll see.
Unless you explicitly want write, I'd leave it at read-only anyway since they had a (very, very quickly corrected) bug that could result in data loss for external libraries not mounted as read-only this last week.