build(deps): 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] <[email protected]>

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20
+FROM alpine:3.21
 
 ARG RUBYGEM_R10K=5.0.0
 ARG RUBYGEM_PUPPET=8.10.0