Update base image

Noticed that there still was an issue starting the backend when doing a clean Docker build. This was because of the following:

* The default Python version on Alpine 3.19 is 3.11.
* This means that system Python packages will be installed into `/usr/lib/python3.11/site-packages`.
* We run Python 3.12 and are expecting Xapian to be installed into `/usr/lib/python3.12/site-packages`.

Alternatively, we might have been able to change `PYTHONPATH` to `/usr/lib/python3.11/site-packages`, but updating to the latest Alpine version makes sense anyway.

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.12-alpine3.19
+FROM python:3.12-alpine3.21
 
 RUN apk --update add \
     build-base \