docs: specify supported binarySource modes (#3138)

lib/config/definitions.js

@@ -68,7 +68,8 @@ const options = [
   },
   {
     name: 'binarySource',
-    description: 'Where to source binaries like `npm` and `yarn` from',
+    description:
+      'Where to source binaries like `npm` and `yarn` from, choices are `bundled`, `global` and `docker`',
     admin: true,
     type: 'string',
     default: 'bundled',

website/docs/self-hosted-configuration.md

@@ -14,6 +14,7 @@ Be cautious when using this option - it will run Renovate over _every_ repositor
 ## binarySource
 
 Set this to 'global' if you wish Renovate to use globally-installed binaries (`npm`, `yarn`, etc) instead of using its bundled versions.
+Set this to 'docker' instead to use docker-based binaries.
 
 ## dryRun