Add an entry for HAB_CTL_SECRET to env-vars docs
@@ -7,6 +7,7 @@ This is a list of all environment variables that can be used to modify the opera
| `HAB_AUTH_TOKEN` | build system | no default | Authorization token used to perform privileged operations against the depot, e.g. uploading packages or keys.
| `HAB_BINLINK_DIR` | build system | `/hab/bin` | Allows you to change the target directory for the symlink created when you run `hab pkg binlink`. The default value is already included in the `$PATH` variable inside the Studio. |
| `HAB_CACHE_KEY_PATH` | build system, Supervisor | `/hab/cache/keys` if running as root; `$HOME/.hab/cache/keys` if running as non-root | Cache directory for origin signing keys |
+| `HAB_CTL_SECRET` | Supervisor | no default | Shared secret used for [communicating with a Supervisor](/docs/using-habitat/#remote-control). |
| `HAB_BLDR_CHANNEL` | build system, Supervisor | `stable` | Set the Habitat Builder channel you are subscribing to, to a specific channel. Defaults to `stable`.
| `HAB_BLDR_URL` | build system, Supervisor | `https://bldr.habitat.sh` | Sets an alternate default endpoint for communicating with Builder. Used by the Habitat build system and the Supervisor |
| `HAB_DOCKER_OPTS` | build system | no default | When running a Studio on a platform that uses Docker (MacOS), additional command line options to pass to the `docker` command. |
