chore(docker): pin turbo to 2.9.6 in pruner stage

Match the version locked in package.json so pruner output is reproducible
across builds.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: waleedlatif1

docker/app.Dockerfile

@@ -17,7 +17,7 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
 FROM base AS pruner
 WORKDIR /app
 
-RUN bun install -g turbo
+RUN bun install -g turbo@2.9.6
 
 COPY . .