Pin alpine to 3.21, last version that packages botan2

Botan2 is end of life, thus it was dropped in Alpine 3.22

docker/biboumi/alpine/Dockerfile

@@ -5,7 +5,7 @@
 # This is the prefered way to build the release image, used by the
 # end users, in production.
 
-FROM docker.io/alpine:latest as builder
+FROM docker.io/alpine:3.21 as builder
 
 RUN apk add --no-cache --virtual .build cmake expat-dev g++ git libidn-dev \
         make postgresql-dev python3 sqlite-dev udns-dev util-linux-dev botan-dev
@@ -25,7 +25,7 @@ RUN git clone https://codeberg.org/poezio/biboumi && \
 
 # ---
 
-FROM docker.io/alpine:latest
+FROM docker.io/alpine:3.21
 
 RUN apk add --no-cache libidn libpq libstdc++ libuuid postgresql-libs \
         sqlite-libs udns expat ca-certificates botan