Merge pull request #377 from buildkite-plugins/toote-expand-vars-clar…

…ification

Add clarifications on volume variables

README.md

@@ -128,7 +128,7 @@ steps:
             - "./dist:/app/dist"
 ```
 
-If you want to use environment variables in the `volumes` element, you will need to activate the (unsafe) option `expand-volume-vars`.
+If you want to use environment variables in the `volumes` element, you will need to activate the (unsafe) option `expand-volume-vars` (and most likely escape it using `$$VARIABLE_NAME`).
 
 ## Environment
 
@@ -548,6 +548,8 @@ When set to true, it will activate interpolation of variables in the elements of
 
 :warning: **Important:** this is considered an unsafe option as the most compatible way to achieve this is to run the strings through `eval` which could lead to arbitrary code execution or information leaking if you don't have complete control of the pipeline
 
+Note that rules regarding [environment variable interpolation](https://buildkite.com/docs/pipelines/environment-variables#runtime-variable-interpolation) apply here. That means that `$VARIABLE_NAME` is resolved at pipeline upload time, whereas `$$VARIABLE_NAME` will be at run time. All things being equal, you likely want to use `$$VARIABLE_NAME` on the variables mentioned in this option.
+
 ### `graceful-shutdown` (optional, run only)
 
 Gracefully shuts down all containers via 'docker-compose stop`.