my question is ... I clearly changed the image, and it's still showing it is Debian based. I did the same thing on a virtual install, w/ a completely new mariadb install, and it also still showed it was a Debian install, instead of an alpine install.
Well, that is a good question. One I really don't have an answer since I don't know....
All it matters (at that time, I was running armhf and change the DB to webhippie) is it worked and I was able to update NC to v20 (or v21) and there was no more "yellow flag".
After linuxserver released the "alpine" version, I swapped the DB from webhippie to linuxserver's and kept the TAG on the YML.
After migrating to aarch64, I just kept using it,
But your question made me check what does it show and I confirm, it shows the same kernel and version of the host:
docker mariaDBLinux b172c62b184c 5.10.17-v8+ #1421 SMP PREEMPT Thu May 27 14:01:37 BST 2021 aarch64 GNU/Linux
aarch64 Host Linux tacho 5.10.17-v8+ #1421 SMP PREEMPT Thu May 27 14:01:37 BST 2021 aarch64
So, maybe the "alpine version" is somewhere "hidden" (?!?) on the dockerfile???