Update nginxinc/nginx-unprivileged Docker tag to v1.28

Author: renovate[bot]

Dockerfile

@@ -17,7 +17,7 @@ RUN python theme_common/scripts/combine_mkdocs_config_yaml.py theme_common/mkdoc
 # build the docs
 RUN mkdocs build
 
-FROM nginxinc/nginx-unprivileged:1.27-alpine as deploy
+FROM nginxinc/nginx-unprivileged:1.28-alpine as deploy
 COPY --from=builder $HOME/application/site/ /usr/share/nginx/html/
 COPY default.conf /etc/nginx/conf.d/