docs: Suggested command breaks startup of playout gateway

The error is that the wrong version of Yarn is installed and corepack
is not enabled.

```
Presence of the "packageManager" field indicates that the project is meant to be used with Corepack, a tool included by default with all official Node.js distributions starting from 16.9 and 14.19.
Corepack must currently be enabled by running corepack enable in your terminal. For more information, check out https://yarnpkg.com/corepack.
```

Instead I leave it set to default and add enviornment variables needed
to make it work.

Author: rjmunro

packages/documentation/docs/user-guide/installation/installing-sofie-server-core.md

@@ -59,7 +59,10 @@ services:
   playout-gateway:
     image: sofietv/tv-automation-playout-gateway:release51
     restart: always
-    command: yarn start -host core -port 3000 -id playoutGateway0
+    environment:
+      DEVICE_ID: playoutGateway0
+      CORE_HOST: core
+      CORE_PORT: '3000'
     networks:
       - sofie
       - lan_access