fix(siteruntime): capture Self before Task.Run in artifact deploy; seed MxGateway connections

- DeploymentManagerActor.HandleDeployArtifacts read the Self property inside its
  Task.Run lambda (line dispatching ApplyArtifactDataConnectionsToDcl). Self is
  backed by the ambient ActorCell, null on a thread-pool thread, so it threw
  'no active ActorContext' — surfaced the first time a data connection is
  deployed via deploy-artifacts. Capture Self into a local first (as Sender
  already was).
- seed-sites.sh: create a shared MxGateway data connection (10.100.0.48:5120)
  on each site and deploy artifacts so the DCL establishes them.
- build.sh: nounset-safe empty-array expansion (bash 3.2).

docker/build.sh

@@ -23,10 +23,12 @@ fi
 
 # Build from repo root (so COPY paths in Dockerfile resolve correctly)
 echo "Building scadabridge:latest image..."
+# Note: ${NUGET_ARGS[@]+...} guards against the "unbound variable" error that
+# set -u raises on bash 3.2 (macOS default) when expanding an empty array.
 docker build \
     -t scadabridge:latest \
     -f "$SCRIPT_DIR/Dockerfile" \
-    "${NUGET_ARGS[@]}" \
+    ${NUGET_ARGS[@]+"${NUGET_ARGS[@]}"} \
     "$REPO_ROOT"
 
 echo "Build complete: scadabridge:latest"