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>

MCP.md

@@ -20,7 +20,7 @@ docker run -d --name kb-mcp \
   -e KB_API_KEY=your-engine-key \
   -e KB_MCP_API_KEY=your-agent-key \
   --restart unless-stopped \
-  gitea.dcglab.co.uk/steve/kb/mcp:latest
+  docker.dcglab.co.uk/kb/mcp:latest
 ```
 
 ## MCP tools