Update README.md

Author: bram-pkg

README.md

@@ -6,6 +6,28 @@
 composer require --dev blamebutton/laravel-docker-builder
 ```
 
+## Configuration
+
+Two environment variables need to be set:
+
+* `DOCKER_NGINX_TAG`
+* `DOCKER_PHP_TAG`
+
+This can be done either by adding them to your `.env` file or passing them to the build command.
+
+### Option 1: `.env`
+
+```
+DOCKER_NGINX_TAG=laravel-app:nginx
+DOCKER_PHP_TAG=laravel-app:php
+```
+
+### Option 2: CLI
+
+```shell
+DOCKER_NGINX_TAG=laravel-app:nginx DOCKER_PHP_TAG=laravel-app:php vendor/bin/docker-build
+```
+
 ## Usage
 
 Set the `DOCKER_NGINX_TAG` and `DOCKER_PHP_TAG` environment variables and run: