avoid Dockerfile Entrypoint: "file not found" (#3430)

https://www.koskila.net/how-to-fix-exec-user-process-caused-no-such-file-or-directory-in-docker-on-windows/

docker/Dockerfile.s3tests

@@ -28,4 +28,4 @@ ENV \
     S3TEST_CONF="/s3test.conf"
 
 ENTRYPOINT ["/bin/bash", "-c"]
-CMD ["sleep 30 && exec ./virtualenv/bin/nosetests ${NOSETESTS_OPTIONS-} ${NOSETESTS_ATTR:+-a $NOSETESTS_ATTR} ${NOSETESTS_EXCLUDE:+-e $NOSETESTS_EXCLUDE}"]
+CMD ["sleep 30 && exec ./virtualenv/bin/nosetests ${NOSETESTS_OPTIONS-} ${NOSETESTS_ATTR:+-a $NOSETESTS_ATTR} ${NOSETESTS_EXCLUDE:+-e $NOSETESTS_EXCLUDE}"]