Update alpine Docker tag to v3.22

| datasource | package | from | to   |
| ---------- | ------- | ---- | ---- |
| docker     | alpine  | 3.21 | 3.22 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

build/Dockerfile.nginxplus

@@ -4,7 +4,7 @@ FROM scratch AS nginx-files
 # the following links can be replaced with local files if needed, i.e. ADD --chown=101:1001 <local_file> <container_file>
 ADD --link --chown=101:1001 https://cs.nginx.com/static/keys/nginx_signing.rsa.pub nginx_signing.rsa.pub
 
-FROM alpine:3.21
+FROM alpine:3.22
 
 ARG NGINX_PLUS_VERSION=R34
 # renovate: datasource=github-tags depName=nginx/agent