docs: add docker-compose build (#155)

Author: dunglas

README.md

@@ -7,8 +7,9 @@ A [Docker](https://www.docker.com/)-based installer and runtime for the [Symfony
 ## Getting Started
 
 1. If not already done, [install Docker Compose](https://docs.docker.com/compose/install/)
-2. Run `docker-compose up` (the logs will be displayed in the current shell)
-3. Open `https://localhost` in your favorite web browser and [accept the auto-generated TLS certificate](https://stackoverflow.com/a/15076602/1352334)
+2. Run `docker-compose build --pull --no-cache` to build fresh images
+3. Run `docker-compose up` (the logs will be displayed in the current shell)
+4. Open `https://localhost` in your favorite web browser and [accept the auto-generated TLS certificate](https://stackoverflow.com/a/15076602/1352334)
 
 ## Features