Docs: VDI prerequisites — Docker install and usermod instructions

Installation guide and VDI docs page now document that Docker must be
installed separately and rustguac user added to docker group manually.

docs/installation.md

@@ -40,6 +40,21 @@ sudo /opt/rustguac/bin/drive-setup.sh
 
 See [Drive / File Transfer](integrations.md#drive--file-transfer--luks-encryption) for details.
 
+5. **(Optional) Enable VDI desktop containers:**
+
+If you want to use VDI sessions (ephemeral Docker desktop containers), install Docker and grant rustguac access:
+
+```bash
+# Install Docker (if not already installed)
+curl -fsSL https://get.docker.com | sh
+
+# Allow rustguac to manage containers
+sudo usermod -aG docker rustguac
+sudo systemctl restart rustguac
+```
+
+Then add a `[vdi]` section to your config — see [VDI Desktop Containers](vdi.md) for full setup.
+
 ## Option B: Bare-metal install script
 
 For fresh Debian 13 systems, the install script builds everything from source:

docs/vdi.md

@@ -12,6 +12,21 @@ rustguac can spawn ephemeral Docker desktop containers on demand. Each user gets
 6. On logout from the desktop, the container is stopped and removed
 7. Idle containers (no active session) are automatically cleaned up after a configurable timeout
 
+## Prerequisites
+
+VDI requires Docker on the same machine as rustguac. Install Docker and grant access:
+
+```bash
+# Install Docker (if not already installed)
+curl -fsSL https://get.docker.com | sh
+
+# Allow the rustguac user to manage containers
+sudo usermod -aG docker rustguac
+sudo systemctl restart rustguac
+```
+
+You also need at least one Docker image with xrdp pre-pulled on the host (see [Docker image requirements](#docker-image-requirements) below).
+
 ## Configuration
 
 Add a `[vdi]` section to your config file: