Update docs to include cap_add, cap_drop, devices

| `tls_cert_path`             | when set it will use `ca.pem`, `cert.pem` and `key.pem` from that folder to make secure TLS connection to Docker (useful in boot2docker) |

| `image`                     | use this image to run builds |

| `privileged`                | make container run in Privileged mode (insecure) |

| `cap_add`                   | add additional Linux capabilities to the container |

| `cap_drop`                  | drop additional Linux capabilities from the container |

| `devices`                   | share additional host devices with the container |

| `disable_cache`             | disable automatic |

| `wait_for_services_timeout` | specify how long to wait for docker services, set to 0 to disable, default: 30 |

| `cache_dir`                 | specify where Docker caches should be stored (this can be absolute or relative to current working directory) |

  tls_cert_path = "/Users/ayufan/.boot2docker/certs"

  image = "ruby:2.1"

  privileged = false

  cap_add = ["NET_ADMIN"]

  cap_drop = ["DAC_OVERRIDE"]

  devices = ["/dev/net/tun"]

  disable_cache = false

  wait_for_services_timeout = 30

  cache_dir = ""

On the other hand there's privileged mode which enables full access to host system, permission to mount and umount volumes and run nested containers. It's not advised to run containers in privileged mode.

More granular permissions can be configured in non-privileged mode via the `cap_add`/`cap_drop` settings.

## Systems with Docker installed

**This applies to installations below 0.5.0 or one's that were upgraded to newer version**