build(deps): bump alpine from 3.20.3 to 3.21.0 in /Containers/borgbackup

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>

Containers/borgbackup/Dockerfile

@@ -1,5 +1,5 @@
 # syntax=docker/dockerfile:latest
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 
 RUN set -ex; \
     \