[FAQ Bot] NEW: Why did Render deploy the wrong service from my multi-stage Dockerfile?

_questions/llm-zoomcamp/project/031_5b343ea5b3_render-wrong-service-multistage-dockerfile.md

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+---
+id: 5b343ea5b3
+question: Why did Render deploy the wrong service from my multi-stage Dockerfile?
+sort_order: 31
+---
+
+Render builds your Dockerfile with no `--target` flag (its Blueprint spec has no field to select a build stage), and a target-less `docker build` always produces the stage declared last. If your `Dockerfile` has multiple stages — e.g. an `api` stage and a `ui` stage sharing a base — whichever comes last is what Render runs. Reordering stages or appending a new one silently changes what gets deployed: the build still succeeds, it just ships the wrong image.
+
+This is easy to miss locally because `docker-compose` sets `target:` per service, so the same Dockerfile behaves correctly under compose while deploying the wrong thing on Render:
+
+```dockerfile
+FROM python:3.11-slim AS deps
+WORKDIR /app
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+COPY . .
+
+# UI stage — deliberately NOT last
+FROM deps AS ui
+EXPOSE 8501
+CMD ["streamlit", "run", "app.py", "--server.address=0.0.0.0", "--server.port=8501"]
+
+# `api` is deliberately the LAST stage: Render builds with no --target,
+# and a target-less build resolves to whichever stage comes last.
+# DO NOT append new stages below this one.
+FROM deps AS api
+EXPOSE 8000
+CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]
+```
+
+```yaml
+services:
+  api:
+    build: { context: ., target: api }
+  ui:
+    build: { context: ., target: ui }
+```
+
+To keep it fixed: keep the deployed service as the last stage and verify by hitting a route unique to that service (not a shared `/health` endpoint both services might answer identically). Before pushing, reproduce Render's target-less build locally:
+
+```bash
+docker build -t check .  # no --target, same as Render
+docker run --rm -p 8010:8000 check
+curl -s localhost:8010/health
+```
+
+See [Render's Docker docs](https://docs.render.com/docker) — multi-stage builds are supported, but there is no target-stage selection.