Improve docs about custom configurations

README.md

@@ -62,7 +62,12 @@ It is composed of two pieces:
 - an HTTP server running at http://172.17.42.1:14242 that serves the private keys;
 - a bash script `ONVAULT` that need to be installed in the image to allow accessing the private keys during the build process.
 
-The `ONVAULT` script accepts a custom URI for the vault server through the environment variable `VAULT_URI`.
+### Custom configurations
+
+Some custom configurations are allowed through environment variables
+
+- `VAULT_URI`: custom URI for the vault server
+- `VAULT_SSH_KEY`: custom ssh key name used during `ONVAULT` command
 
 ### The private keys server