docker: gateway: bump alpine from 3.20.3 to 3.21.0 in /gateway

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] <[email protected]>

gateway/Dockerfile

@@ -56,7 +56,7 @@ WORKDIR $GOPATH/src/github.com/shellhub-io/shellhub/gateway
 ENTRYPOINT ["/entrypoint.sh"]
 
 # production stage
-FROM alpine:3.20.3 AS production
+FROM alpine:3.21.0 AS production
 
 RUN apk add libgcc curl certbot certbot-nginx