docs: refactor and add a doc explaining how to deploy in prod

docs/build.md

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+# 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.
+
+    $ SERVER_NAME=symfony.wip docker-compose up --build