Vdi port ranges and hooks

[vk2amv]

Adds VDI container lifecycle hooks and optional host port range control.

This change lets operators configure a bounded localhost port range for Docker-published VDI RDP ports. When configured, Rustguac selects ports from the specified inclusive range instead of asking Docker for an arbitrary random port. Existing containers whose mapped RDP port falls outside the configured range are replaced so the setting is consistently enforced.

It also adds an optional VDI container hook script. When configured, Rustguac calls the hook after it has inspected the container and learned the mapped RDP port, but before it begins readiness checks. The hook is called again before container removal. This gives deployments a supported way to run external preparation and cleanup logic around the container’s mapped RDP endpoint.

New [vdi] options:
port_range_start = 39000
port_range_end = 39999
container_hook_script = "/opt/rustguac/vdi-container-hook.sh"
container_hook_timeout_secs = 10

Hook invocation format:

<script> up <script> down The hook also receives equivalent environment variables: RUSTGUAC_VDI_HOOK_ACTION RUSTGUAC_VDI_PORT RUSTGUAC_VDI_CONTAINER_ID RUSTGUAC_VDI_CONTAINER_NAME