build(deps): bump alpine from 3.20.3 to 3.21.0

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>

Dockerfile

@@ -34,7 +34,7 @@ RUN go build -v \
 
 # https://github.com/hadolint/hadolint/issues/861
 # hadolint ignore=DL3029
-FROM --platform=${TARGETPLATFORM:-linux/amd64} alpine:3.20.3
+FROM --platform=${TARGETPLATFORM:-linux/amd64} alpine:3.21.0
 
 COPY --from=build /go/src/github.com/enix/x509-certificate-exporter/x509-certificate-exporter /x509-certificate-exporter