nginx.conf

user  nginx;
worker_processes  1;

error_log /var/log/nginx/error.log debug;
pid /var/run/nginx.pid;

# Need perl for managing the environment variables
load_module "modules/ngx_http_perl_module.so";

# Expose environment variables that we need
env MESSAGE;
env MESSAGE_FILE;
env TITLE;

events {
    worker_connections  1024;
}

http {
    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log /var/log/nginx/access.log  main;

    sendfile on;

    keepalive_timeout 65;

    charset UTF-8;
    gzip off;

    # Extract title from configuration
    perl_set $title 'sub {
      if(defined $ENV{"TITLE"}) {
        $ENV{"TITLE"}
      } else {
        "TITLE"
      }
    }';

    # Extract message from configuration
    perl_set $message 'sub {
      use strict;
      use warnings;

      if(defined $ENV{"MESSAGE"}) {
        return $ENV{"MESSAGE"};
      } elsif(defined $ENV{"MESSAGE_FILE"}) {
        local $/ = undef;
        my $message_file = $ENV{"MESSAGE_FILE"};
        my $file = "/web/" . $message_file;
        open my $fh, "<", $file
          or die "could not open $file: $!";
        return <$fh>;
      } else {
        return "MESSAGE or MESSAGE_FILE";
      }
    }';

    server {
        listen       80;
        server_name  localhost;
        sendfile  off;

        root   /usr/share/nginx/html;

        # Put title and message in placeholders
        sub_filter '<!-- MESSAGE -->' '$message';
        sub_filter '<!-- TITLE -->' '$title';
        sub_filter_once on;

        location / {
            if ($uri !~ ^/maintenance_files/) {
                # Rewrite everything not in maintenance_files otherwise the image
                # will be rewritten.
                rewrite (.*) /index.html last;
            }
            expires           0;
            add_header        Cache-Control private;
        }

        location /index.html {
            index  index.html;
        }
    }
}