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>

release-engine.sh

@@ -15,9 +15,9 @@ ENGINE_DIR="$SCRIPT_DIR/engine"
 VERSION_FILE="$ENGINE_DIR/VERSION"
 
 # Container registry
-REGISTRY="${REGISTRY:-gitea.dcglab.co.uk}"
-IMAGE_ORG="${IMAGE_ORG:-steve}"
-IMAGE_BASE="${REGISTRY}/${IMAGE_ORG}/kb"
+REGISTRY="${REGISTRY:-docker.dcglab.co.uk}"
+IMAGE_ORG="${IMAGE_ORG:-}"
+IMAGE_BASE="${REGISTRY}${IMAGE_ORG:+/${IMAGE_ORG}}/kb"
 
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