Bump alpine from 3.20.3 to 3.21.0 in /.ci/releaser/alpine

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] <[email protected]>

.ci/releaser/alpine/Dockerfile

@@ -1,6 +1,6 @@
 # Initially copied from
 # https://github.com/jordansissel/fpm/blob/master/Dockerfile
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 
 SHELL ["/bin/ash", "-eo", "pipefail", "-c"]