langchain[minor]: add libs/partners to dev.Dockerfile (#21902)

Resolves #21886 by adding "COPY libs/partners ../partners/" to
libs/dev.Dockerfile

Twitter: @kabakongo

libs/langchain/dev.Dockerfile

@@ -53,5 +53,8 @@ COPY libs/community/ ../community/
 # Copy the text-splitters library for installation
 COPY libs/text-splitters/ ../text-splitters/
 
+# Copy the partners library for installation
+COPY libs/partners ../partners/
+
 # Install the Poetry dependencies (this layer will be cached as long as the dependencies don't change)
 RUN poetry install --no-interaction --no-ansi --with dev,test,docs