doc: Update Custom Configurarion documentation

README.md

@@ -21,6 +21,7 @@ For other methods to install `salt-master` please refer to the [Official Salt Pr
   - [Changelog](CHANGELOG.md)
 - [Quick Start](#quick-start)
 - [Configuration](#configuration)
+  - [Custom Configuration](#custom-configuration)
   - [Custom States](#custom-states)
   - [Minion Keys](#minion-keys)
   - [Master Signed Keys](#master-signed-keys)
@@ -99,6 +100,39 @@ docker run --name salt_master --detach \
 
 ## Configuration
 
+### Custom Configuration
+
+This image uses its own `master.yml` file to configure `salt-master` to run properly inside the container. However, you can still tune other configuration parameters to fit your needs by adding your configuration files into a `config/` directory and mounting it into `/home/salt/data/config/`.
+
+For example, you can customize the [Reactor System](https://docs.saltproject.io/en/latest/topics/reactor/index.html) by adding a `reactor.conf` file to `config/`:
+
+```sls
+# config/reactor.conf
+reactor:                        # Master config section "reactor"
+  - 'salt/minion/*/start':      # Match tag "salt/minion/*/start"
+    - /srv/reactor/start.sls    # Things to do when a minion starts
+```
+
+Then, you have to add the `start.sls` file into your `roots/reactor/` directory:
+
+```sls
+# roots/reactor/start.sls
+highstate_run:
+  local.state.apply:
+    - tgt: {{ data['id'] }}
+```
+
+Finally, run your `docker-salt-master` instance mounting the required directories:
+
+```sh
+docker run --name salt_master -d \
+    --publish 4505:4505 --publish 4506:4506 \
+    --volume $(pwd)/roots/:/home/salt/data/srv/ \
+    --volume $(pwd)/keys/:/home/salt/data/keys/ \
+    --volume $(pwd)/config/:/home/salt/data/config/ \
+    cdalvaro/docker-salt-master:latest
+```
+
 ### Custom States
 
 In order to provide salt with your custom states you must mount the volume `/home/salt/data/srv/` with your `roots` directory inside it.
@@ -555,7 +589,7 @@ Where `salt-service` is one of: `salt-master` os `salt-api` (if `SALT_API_SERVIC
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 [arch_link]: https://github.com/users/cdalvaro/packages/container/package/docker-salt-master