Bump library/alpine from 3.19.4 to 3.21.2

Bumps library/alpine from 3.19.4 to 3.21.2.

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

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

Author: dependabot[bot]

Dockerfile

@@ -66,7 +66,7 @@ ARG RUNTIME_DEPS="\
     tzdata \
     "
 
-FROM docker.io/library/alpine:3.19.4 as builder
+FROM docker.io/library/alpine:3.21.3 as builder
 
 ARG HUMHUB_VERSION
 ARG BUILD_DEPS
@@ -91,7 +91,7 @@ RUN composer config --no-plugins allow-plugins.yiisoft/yii2-composer true && \
     grunt build-assets && \
     rm -rf ./node_modules
 
-FROM docker.io/library/alpine:3.19.4 as base
+FROM docker.io/library/alpine:3.21.3 as base
 
 ARG HUMHUB_VERSION
 ARG RUNTIME_DEPS