build(deps): bump alpine from 3.20.3 to 3.21.0 in /Containers/imaginary

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] <support@github.com>

Containers/imaginary/Dockerfile

@@ -13,7 +13,7 @@ RUN set -ex; \
         build-base; \
     go install github.com/h2non/imaginary@"$IMAGINARY_HASH";
 
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 RUN set -ex; \
     apk upgrade --no-cache -a; \
     apk add --no-cache \