feat: add optional OCI image version to container state attributes

[RF1705]

Title: Add optional OCI image version attribute to container sensors

## Summary

This PR exposes the optional OCI image version from Dockge service data as a Home Assistant container sensor attribute.

When Dockge provides a service-level `imageVersion` value, the integration now publishes it as:

```yaml
image_version: 2026.5.4

Why

Many container images use stable or generic tags, while the real application version is stored in the OCI image label:

org.opencontainers.image.version

For example:

image: ghcr.io/home-assistant/home-assistant:stable
image_tag: stable
image_version: 2026.5.4

This gives Home Assistant users a more useful version value without losing the existing image tag.

Changes

• Adds image_version to DockgeContainerSensor attributes.
• Reads the value from Dockge’s optional imageVersion service field.
• Updates the README container sensor description.
• Keeps existing behavior unchanged when imageVersion is not present.

Compatibility

This is backward-compatible with older Dockge API versions.

If Dockge does not provide imageVersion, the attribute is simply null, while existing attributes such as image, image_name, and image_tag continue to work as before.

Example

Before:

image: ghcr.io/home-assistant/home-assistant:stable
image_name: ghcr.io/home-assistant/home-assistant
image_tag: stable

After:

image: ghcr.io/home-assistant/home-assistant:stable
image_name: ghcr.io/home-assistant/home-assistant
image_tag: stable
image_version: 2026.5.4

Testing

Validated Python syntax with:

PYTHONPYCACHEPREFIX=/private/tmp/dockge-ha-pycache python3 -m compileall custom_components/dockge