fix(docker): consume CACHE_BUST arg so BuildKit invalidates cache

ARG alone doesn't invalidate layers on BuildKit unless referenced.
Add a RUN that echoes the value before the expensive WASM build step.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -68,8 +68,10 @@ RUN cargo build --profile dist --bin ironclaw
 
 # Stage 4b: Build all WASM extensions from source (only used by runtime-staging)
 FROM builder AS wasm-builder
+ARG CACHE_BUST
 
 RUN apt-get update && apt-get install -y --no-install-recommends jq && rm -rf /var/lib/apt/lists/*
+RUN echo "cache-bust=${CACHE_BUST}"
 
 RUN set -eux; \
     mkdir -p /app/wasm-bundles/tools /app/wasm-bundles/channels; \