chore(deps): bump alpine from 3.20 to 3.21 (#540)

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

@@ -38,7 +38,7 @@ RUN CGO_ENABLED=1 \
     go build -o post-renderer ./cmd/post-renderer
 
 # Final image
-FROM --platform=${TARGETPLATFORM:-linux/amd64} alpine:3.20
+FROM --platform=${TARGETPLATFORM:-linux/amd64} alpine:3.21
 ARG TARGETPLATFORM
 # kubectl url has no variant (v7)
 # helm url has dashes and no variant (v7)