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>

README.md

@@ -33,7 +33,7 @@ docker run -d --name kb-engine \
   -e KB_DEVICE=auto \
   -e KB_API_KEY=your-secret-key \
   --restart unless-stopped \
-  gitea.dcglab.co.uk/steve/kb/engine:latest-nvidia
+  docker.dcglab.co.uk/kb/engine:latest-nvidia
 
 # CPU only (no GPU required — smaller image)
 docker run -d --name kb-engine \
@@ -42,7 +42,7 @@ docker run -d --name kb-engine \
   -e KB_MODEL=all-MiniLM-L6-v2 \
   -e KB_API_KEY=your-secret-key \
   --restart unless-stopped \
-  gitea.dcglab.co.uk/steve/kb/engine:latest-cpu
+  docker.dcglab.co.uk/kb/engine:latest-cpu
 ```
 
 Or use a compose file from the repo: