How to pass the content of a file to Helm values.yaml
You can't use Helm templating in the values.yaml file. (Unless the chart author has specifically called the tpl function when the value is used; for this variable it doesn't, and that's usually called out in the chart documentation.)
Your two options are to directly embed the file content in the values.yaml file you're passing in, or to use the Helm --set-file option (link to v2 docs).
helm install --dry-run --debug \ stable/rabbitmq \ --name testrmq \ --namespace rmq \ -f rabbit-values.yaml \ --set-file rabbitmq.advancedConfig=rabbitmq.configThere isn't a way to put a file pointer inside your local values YAML file though.