Add additional ARG version line in Dockerfile (#423)

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**What this PR does / why we need it**:
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In #404, Merlin SDK Dockerfile misses ARG version.

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**Checklist**

- [ ] Added unit test, integration, and/or e2e tests
- [x] Tested locally
- [ ] Updated documentation
- [ ] Update Swagger spec if the PR introduce API changes
- [ ] Regenerated Golang and Python client if the PR introduce API
changes

python/sdk/Dockerfile

@@ -10,5 +10,7 @@ RUN apt-get update && apt-get install build-essential curl vim wget -y
 
 COPY . .${WORKDIR}
 
+ARG VERSION
+
 RUN pip install merlin-sdk==${VERSION}
 RUN pip install merlin-sdk[test]==${VERSION}