feat: add mount-docker-socket option (#749)

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

README.md

@@ -12,6 +12,7 @@ GitHub Action to run Renovate self-hosted.
 - [Options](#options)
   - [`configurationFile`](#configurationfile)
   - [`env-regex`](#env-regex)
+  - [`mount-docker-socket`](#mount-docker-socket)
   - [`token`](#token)
   - [`renovate-image`](#renovate-image)
   - [`renovate-version`](#renovate-version)
@@ -59,6 +60,13 @@ If you want to use this with just the single configuration file, make sure to in
 Allows to configure the regex to define which environment variables are passed to the renovate container.
 See [Passing other environment variables](#passing-other-environment-variables) section for more details.
 
+## `mount-docker-socket`
+
+Default to `false`. If set to `true` the action will mount the Docker socket
+inside the renovate container so that the commands can use Docker. Can be useful
+for `postUpgradeTasks`'s commands. Also add the user inside the renovate
+container to the docker group for socket permissions.
+
 ### `token`
 
 [Generate a personal access token](https://github.com/settings/tokens), with the `repo:public_repo` scope for only public repositories or the `repo` scope for public and private repositories, and add it to _Secrets_ (repository settings) as `RENOVATE_TOKEN`. You can also create a token without a specific scope, which gives read-only access to public repositories, for testing. This token is only used by Renovate, see the [token configuration](https://docs.renovatebot.com/self-hosted-configuration/#token), and gives it access to the repositories. The name of the secret can be anything as long as it matches the argument given to the `token` option.