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] <support@github.com>

image-api-server.dockerfile

@@ -20,7 +20,7 @@ RUN go build -v \
     -ldflags "-s -w -extldflags -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now" \
     -o image-api-server cmd/alpacascloud/main.go
 
-FROM alpine:3.20
+FROM alpine:3.21
 
 RUN apk add --no-cache vips