Nginx environment variable substitution with Docker for SaaS apps

Software as a service apps should store configuration values in environment variables when you rely on the twelve-factor app Saas guide. The twelve-factor rule says you should seperate config from your code, because configuration varies across environment stages: https://12factor.net/config

Since nginx version 1.19 it’s possible to replace values in nginx config files with given environment variables. This eases the build of self designed Dockerfiles for your nginx server dramatically. Let’s take a look at a simple example with docker compose:

version: '3.9'
services:
  nginx:
    image: nginx:1.19-alpine
    ports:
      - "8080:80"
    environment:
      FCGI_HOST: php
    volumes:
      - ./:/var/www/html:ro
      - ./docker/vhost.template:/etc/nginx/templates/default.conf.template
  php:
    image: webdevops/php:8.0-alpine
    working_dir: /var/www/html

Our local vhost.template file looks like this:

server {
    server_name  server_name _;
    listen       80 default_server;
    root /var/www/html/web;
    index index.php;

    location / {
        try_files $uri /index.php$is_args$args;
    }

    location ~ \.php$ {
       fastcgi_pass ${FCGI_HOST}:9000;
       fastcgi_split_path_info ^(.+\.php)(/.*)$;
       include…