build(deps): bump library/alpine in /packaging/docker/alpine

Bumps library/alpine from 3.20 to 3.21.

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

Signed-off-by: dependabot[bot] <support@github.com>

packaging/docker/alpine/Dockerfile

@@ -4,7 +4,7 @@ ADD . /go/src/app
 WORKDIR /go/src/app
 RUN make miniflux
 
-FROM docker.io/library/alpine:3.20
+FROM docker.io/library/alpine:3.21
 
 LABEL org.opencontainers.image.title=Miniflux
 LABEL org.opencontainers.image.description="Miniflux is a minimalist and opinionated feed reader"