Update docker.io/alpine Docker tag to v3.21.3 (#51)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| docker.io/alpine | stage | patch | `3.21.2` -> `3.21.3` |

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Dockerfile

@@ -20,7 +20,7 @@ ARG snapshot_versions_type=prod
 
 # Use alpine instead of busybox here, because busybox's wget can't verify TLS
 # certificate.
-FROM docker.io/alpine:3.21.2@sha256:56fa17d2a7e7f168a043a2712e63aed1f8543aeafdcee47c58dcffe38ed51099 as snapshot-versions-getter-base
+FROM docker.io/alpine:3.21.3@sha256:a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c as snapshot-versions-getter-base
 
 # Remove the community repository. This is to ensure that we only rely on
 # packages with future availability.