Bump alpine from 3.20.3 to 3.21.3 in /docker

Bumps alpine from 3.20.3 to 3.21.3.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20.3
+FROM alpine:3.21.3
 
 RUN apk add --no-cache gettext curl nginx php83 php83-fpm php83-opcache php83-pdo php83-pdo_sqlite php83-openssl && \
     mkdir /var/www/html