Bump alpine from 3.20 to 3.21 in /client

Bumps alpine from 3.20 to 3.21.

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

client/Dockerfile

@@ -1,6 +1,6 @@
 FROM nginxproxy/forego:0.18.2-debian AS forego
 
-FROM alpine:3.20
+FROM alpine:3.21
 
 COPY --from=forego /usr/local/bin/forego /usr/local/bin/forego