Restore CMD instruction for x509_updater.py in Dockerfile

Author: ivukotic

x509_secrets/Dockerfile

@@ -28,5 +28,4 @@ RUN echo "Timestamp:" `date --utc` | tee /image-build-info.txt
 
 ENV X509_USER_PROXY /tmp/x509up
 
-# CMD sh -c "python3 x509_updater.py --voms $VOMS"
-CMD ["python"]
+CMD sh -c "python3 x509_updater.py --voms $VOMS"