Bump alpine from 3.20 to 3.21

Bumps alpine from 3.20 to 3.21.

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Dockerfile.alpine

@@ -1,7 +1,7 @@
 # Copyright (c) 2018 kalaksi@users.noreply.github.com.
 # This work is licensed under the terms of the MIT license. For a copy, see <https://opensource.org/licenses/MIT>.
 
-FROM alpine:3.20
+FROM alpine:3.21
 LABEL maintainer="kalaksi@users.noreply.github.com"
 
 # See tinyproxy.conf for better explanation of these values.