Add note regarding Dot Env custom file (#460)

Relates to getsentry/self-hosted#1113.

src/docs/self-hosted/index.mdx

@@ -22,7 +22,7 @@ You very likely will want to adjust the default configuration for Sentry. These
 
 1. **`sentry/sentry.conf.py`**—Contains more advanced configuration. This file is generated from [`sentry/sentry.conf.example.py`](https://github.com/getsentry/self-hosted/blob/master/sentry/sentry.conf.example.py) during installation.
 
-1. **Environment variables**—The available keys are defined in [.env](https://github.com/getsentry/self-hosted/blob/master/.env). Use some system-dependent means of setting environment variables if you need to override any of them.
+1. **Environment variables**—The available keys are defined in [.env](https://github.com/getsentry/self-hosted/blob/master/.env). Use some system-dependent means of setting environment variables if you need to override any of them. To avoid Git changes, simply create a file called `.env.custom` and insert your system-dependent environment variables there. In order to use this, please use `docker-compose --env-file /path/to/.env.custom up -d`.
 
 1. [Geolocation](/self-hosted/geolocation/) uses **a custom configuration file** to conform to the underlying technology.