# Build Options ## Selecting a Specific Symfony Version Use the `SYMFONY_VERSION` environment variable to select a specific Symfony version. For instance, use the following command to install Symfony 4.4: $ SYMFONY_VERSION=4.4.* docker-compose up --build To install a non-stable version of Symfony, use the `STABILITY` environment variable during the build. The value must be [a valid Composer stability option](https://getcomposer.org/doc/04-schema.md#minimum-stability)) . For instance, use the following command to use the `master` branch of Symfony: $ STABILITY=dev docker-compose up --build ## Customizing the Server Name Use the `SERVER_NAME` environment variable to define your custom server name(s). $ SERVER_NAME="symfony.wip, caddy:80" docker-compose up --build If you use Mercure, keep `caddy:80` in the list to allow the PHP container to request the caddy service.