Add Backends

Backends.md

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+## `docker`
+
+The Docker backend integrates with a docker daemon running on your system to provide shell access into containers.
+
+### Settings
+
+- **command** (optional, []string): The command to run inside the container.  Default is `["/bin/sh"]`.
+- **privileged** (optional, bool): If `true`, the command will run in privileged mode. Default is `false`.
+- **user** (optional, string): The user to run the command as inside the container.
+
+### Permissions
+
+The docker backend exposes container names to the permissions system. For example, if you wanted to allow `group1` access to all containers beginning with `g1`, you could do:
+
+```hcl
+permissions = {
+  group1 = {
+    allow = ["g1*"]
+  }
+}
+```
+
+## `nomad`
+
+The Nomad backend integrates with a [Nomad](https://nomadproject.io) server to provide shell access into Nomad allocations.
+
+### Settings
+
+- **server** (string): The Nomad server address. 
+- **delimiter** (optional, string): The delimiter used for the Seashell argument . Default is `"."`.
+- **region** (optional, string): The Nomad region.
+- **namespace** (optional, string): The Nomad namespace.
+- **auth_token** (optional, string): The authentication token for Nomad.
+- **command** (optional, []string): The command to run within the Nomad allocation. Default is `["/bin/sh"]`.
+
+### Permissions
+
+The nomad backend exposes job, task, and task group names to the permissions system. For example, if you wanted to allow `group1` to access the `hello-world` job and the `hello` task group, but not the `hello1` task within that task group, you could do:
+
+```hcl
+permissions = {
+  group1 = {
+    allow = ["job:hello-world", "group:hello"]
+    deny = ["task:hello1"]
+  }
+}
+```
+
+## `serial`
+
+The Serial backend provides shell access over serial connections.
+
+### Settings
+
+- **directory** (optional, string): The directory where serial files are located (e.g. `/dev`). Either **directory** or **file** must be provided.
+- **file** (optional, string): The specific serial file to use. This setting overrides **directory**.
+- **delimiter** (optional, string): The delimiter used for the Seashell argument . Default is `"."`.
+- **baud_rate** (optional, int): The baud rate for the serial connection. If this is not set, the user will have to provide the baud rate in their `ssh` command.
+- **config** (optional, string): The mode string for the serial connection (e.g. `8n1`). If this is not set, the user will have to provide the mode string in their `ssh` command.
+
+### Permissions
+
+If the **directory** setting is used, the serial backend will expose filenames to the permissions system. For example, to only allow `group1` access to `ttyUSB0`, you could use:
+
+```hcl
+permissions = {
+  group1 = {
+    allow = ["ttyUSB0"]
+  }
+}
+```
+
+## `proxy`
+
+The Proxy backend allows SSH connections to be proxied through another server.
+
+### Settings
+
+- **server** (string): The target server's address.
+- **user** (optional, string): The user to connect as on the proxy server. If this is not set, **userMap** will be used to determine the right user, or the seashell username will be used if the user doesn't exist in the map.
+- **privkey** (optional, string): The path to the private key for authentication. If this is not provided, users will be asked for the target server's password when they attempt to connect.
+- **userMap** (optional, map[string]string): A map from seashell usernames to target server usernames.
+
+### Permissions
+
+The proxy backend doesn't expose any items to the permissions system. You can allow or deny access to the entire route using a wildcard:
+
+```hcl
+permissions = {
+  admins = {
+    allow = ["*"]
+  }
+}
+```