chore(README.md): document CODER_BOOTSTRAP_SCRIPT (#130)

Adds missing docs regarding CODER_BOOTSTRAP_SCRIPT in README
Co-authored-by: Cian Johnston <[email protected]>

Author: bjornrobertsson

README.md

@@ -19,6 +19,8 @@ The environment variables can be used to configure various aspects of the inner
 | `CODER_INNER_HOSTNAME`         | The hostname to use for the inner container.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | false    |
 | `CODER_IMAGE_PULL_SECRET`      | The docker credentials to use when pulling the inner container. The recommended way to do this is to create an [Image Pull Secret](https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/#create-a-secret-by-providing-credentials-on-the-command-line) and then reference the secret using an [environment variable](https://kubernetes.io/docs/tasks/inject-data-application/distribute-credentials-secure/#define-container-environment-variables-using-secret-data). See below for example. | false    |
 | `CODER_DOCKER_BRIDGE_CIDR`     | The bridge CIDR to start the Docker daemon with.                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | false    |
+| `CODER_BOOTSTRAP_SCRIPT`       | The script to use to bootstrap the container. This should typically install and start the agent.                                                                                                                                                                                                                                                                                                                                                                                                                               |          |
+| false                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |          |
 | `CODER_MOUNTS`                 | A list of mounts to mount into the inner container. Mounts default to `rw`. Ex: `CODER_MOUNTS=/home/coder:/home/coder,/var/run/mysecret:/var/run/mysecret:ro`                                                                                                                                                                                                                                                                                                                                                                  | false    |
 | `CODER_USR_LIB_DIR`            | The mountpoint of the host `/usr/lib` directory. Only required when using GPUs.                                                                                                                                                                                                                                                                                                                                                                                                                                                | false    |
 | `CODER_INNER_USR_LIB_DIR`      | The inner /usr/lib mountpoint. This is automatically detected based on `/etc/os-release` in the inner image, but may optionally be overridden.                                                                                                                                                                                                                                                                                                                                                                                 | false    |