Bump alpine from 3.20.3 to 3.21.0

Bumps alpine from 3.20.3 to 3.21.0.

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

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

Dockerfile

@@ -2,7 +2,7 @@
 # https://www.cicirello.org
 # Source repository: https://github.com/cicirello/alpine-plus-plus
 # Source licensed under the MIT License: https://github.com/cicirello/alpine-plus-plus/blob/master/LICENSE
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 LABEL maintainer="development@cicirello.org" \
     org.opencontainers.image.description="A lightweight docker image for shell scripting and git, \
     consisting of Alpine Linux with the addition of bash, coreutils, findutils, gawk, and git." \