add doc on cloud-init support for containers and VMs

Signed-off-by: Paul Gaiduk <paulg@zededa.com>

docs/CLOUD-INIT.md

@@ -0,0 +1,46 @@
+# Cloud-Init
+
+EVE supports [cloud-init](https://cloudinit.readthedocs.io/en/latest/) for configuring VMs and containers. The user-data is passed through the corresponding fields in the [AppInstanceConfig message](https://github.com/lf-edge/eve-api/tree/main/proto/config/appconfig.proto).
+
+## Support in VMs
+
+For VMs, EVE supports cloud-init configuration using the NoCloud datasource. With NoCloud, EVE manually provides both meta-data and user-data received from the controller to the VM. This is done by placing these files on a virtual CD-ROM in the form of an ISO image. Further EVE relies on the cloud-init system present in the ECO's VM image. Upon booting the VM instance, if a cloud-init installation is present, the system checks for these data files, and if found, processes the configurations or scripts defined in them.
+
+For more information on the meta-data consult [ECO-METADATA.md](ECO-METADATA.md).
+
+## Support in Containers
+
+As opposed to the VM implementation, cloud-init in ECO containers does not rely on a cloud-init daemon being present in the container image. Instead, the cloud-init configuration is parsed by EVE and manually applied to the container. EVE's implementation supports two formats for user-data:
+
+- The **legacy** format only supports the definition of environment variables in the form of a simple key-value map. The equal sign "=" is used as delimiter in this case:
+
+    ```text
+    ENV1=value1
+    ENV2=value2
+    ```
+
+- In the **original** cloud-init format the user-data is specified like in any other cloud-init configuration. The current EVE implementation only supports two user-data fields: `runcmd` and `write_files`.
+
+  `runcmd` is only used to set environment variables, similar to the **legacy** format. Trying to use any other command will result in an error. The env definitions **must not** be preceded by an `export` keyword, but it's effect is implied in the implementation.
+
+  `write_files` field supports parameters such as path, content, permissions and encoding. It is used to write one or more files to the container image prior to the container start.
+
+  Every cloud-init configuration **must** begin with the `#cloud-config` header. Example:
+
+    ```yaml
+    #cloud-config
+    runcmd:
+      - ENV1=value1
+      - ENV2=value2
+    write_files:
+      - path: /etc/injected_file.txt
+        permissions: '0644'
+        encoding: b64
+        content: YmxhYmxh
+    ```
+
+## Versioning
+
+Both VM and container implementations support versioning of the user-data. This means that the cloud-init configuration is only reapplied if the version of the user-data has changed. The version is specified in the meta-data file in the `instance-id` field.
+
+If the controller implementation does not support versioning, the user-data will be reapplied each time the version of the `AppInstanceConfig` changes.