Bump alpine from 3.18 to 3.21

Bumps alpine from 3.18 to 3.21.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Dockerfile

@@ -33,7 +33,7 @@ RUN CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH go build -ldflags="-w -s" -o
 #
 # Runtime
 #
-FROM alpine:3.18
+FROM alpine:3.21
 RUN mkdir /app && cd /app
 WORKDIR /app
 COPY --from=backend-builder /app/nuts-registry-admin-demo .