[docker] 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] <[email protected]>

Author: dependabot[bot]

Dockerfile

@@ -1,5 +1,5 @@
 # sample dockerfile for testing call-docker-build.yaml
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 
 RUN apk add --no-cache curl