fix(app/docker): correctly parse input variables (#852)

.github/readme/partials/documentation/setup/docker.md

@@ -2,7 +2,7 @@
 
 ## 0️ Prepare your machine
 
-A server with a recent version of [docker](https://www.docker.com/) is required.
+A machine with a recent version of [docker](https://www.docker.com/) is required.
 
 ## 1️ Run docker image
 
@@ -12,3 +12,7 @@ docker run --env INPUT_TOKEN=**** --env INPUT_USER=user --volume=/tmp:/renders g
 ```
 
 To pass parameters, pass environment variable with the same name as the corresponding action option but in uppercase and prefixed with `INPUT_`.
+
+Generated files will be created in the mounted `/renders` directory.
+
+> 💡 When running *metrics* with docker, [`output_action`](/source/plugins/core/README.md#-configuring-output-action) will automatically default to `none` instead. To use a different output action, both `GITHUB_REPOSITORY` (notice the absence of `INPUT_` prefix) and `INPUT_COMMITTER_TOKEN` (with sufficient permissions) environment variables must be set.