chore: phase 0 reproducible docker/dev setup with version pinning

Dockerfile.lfx

@@ -8,7 +8,10 @@ ENV PYTHONUNBUFFERED=1 \
     PIP_NO_CACHE_DIR=1
 
 # lfx = flow runtime; requests = convenience for custom-component code.
-RUN pip install lfx requests
+# Pinned to the Langflow minor (contract: major.minor must match Langflow;
+# patches independent). Bump together with LANGFLOW_VERSION (.env.example).
+ARG LFX_VERSION=1.12.0
+RUN pip install "lfx~=${LFX_VERSION}" requests
 
 RUN mkdir -p /flows
 VOLUME /flows