chore(container): update alpine docker tag to v3.21.0 (#30164)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| alpine | final | minor | `3.20.3` -> `3.21.0` |
| alpine | stage | minor | `3.20.3` -> `3.21.0` |

---

> [!WARNING]
> Some dependencies could not be looked up. Check the Dependency
Dashboard for more information.

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Signed-off-by: Kjeld Schouten <info@kjeldschouten.nl>
Co-authored-by: Kjeld Schouten <info@kjeldschouten.nl>

containers/apps/nextcloud-imaginary/Dockerfile

@@ -11,7 +11,8 @@ RUN set -ex; \
         vips-poppler; \
     go install github.com/h2non/imaginary@b632dae8cc321452c3f85bcae79c580b1ae1ed84
 
-FROM alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+FROM alpine:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
+
 # hadolint ignore=DL3018
 RUN set -ex; \
     apk add --no-cache \

containers/apps/nextcloud-push-notify/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+FROM alpine:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
 
 SHELL ["/bin/ash", "-eo", "pipefail", "-c"]
 # hadolint ignore=DL3018

containers/apps/traefik/Dockerfile

@@ -1,7 +1,7 @@
 FROM traefik:v3.2.3@sha256:efb87236c8c92599bcd3a67a7a8a55e0f255665f4719722bf398935aa9b92270
 # Above line used by CI to find tags etc
 
-FROM alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a as plugins
+FROM alpine:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45 as plugins
 
 # FIXME: Find a way to keep versions up to date
 # -- Use the following prefixes to specify plugins to clone

containers/base/alpine/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+FROM alpine:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
 
 ARG CONTAINER_NAME
 ARG CONTAINER_VER