Move container registry to docker.dcglab.co.uk/kb

registry.dcglab.co.uk has been decommissioned and replaced by
docker.dcglab.co.uk. Images move off the Gitea built-in registry (which
never successfully accepted a push) to the new host, published at the
root with no org segment: docker.dcglab.co.uk/kb/engine and .../kb/mcp.

IMAGE_ORG is now empty by default but still honoured when set, so the
documented REGISTRY=ghcr.io IMAGE_ORG=myorg override keeps working.

Gitea remains the git forge — the release download URLs in README.md are
unchanged and still point at gitea.dcglab.co.uk.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

DEVELOPER.md

@@ -90,7 +90,7 @@ curl http://localhost:8000/api/v1/status | jq .version
 
 ### Docker images
 
-Images are pushed to `gitea.dcglab.co.uk/steve/kb/engine` with tags:
+Images are pushed to `docker.dcglab.co.uk/kb/engine` with tags:
 
 - `engine-v2.0.6-nvidia` / `engine-v2.0.6-cpu` — versioned
 - `latest-nvidia` / `latest-cpu` — latest release