Add retrying to pip3 install

To enable Python usage of "from retrying import retry" for network
connectivity retries. Utilized in websocket test.

Signed-off-by: Michael Shepos <shepos@us.ibm.com>
Change-Id: Ic64bee439b1f07103960ab9bda66e7383ba227ea
diff --git a/scripts/build-qemu-robot-docker.sh b/scripts/build-qemu-robot-docker.sh
index 8ee4ef3..48c5203 100755
--- a/scripts/build-qemu-robot-docker.sh
+++ b/scripts/build-qemu-robot-docker.sh
@@ -70,6 +70,7 @@
 RUN pip3 install \
     tox \
     requests \
+    retrying \
     json2yaml \
     robotframework \
     robotframework-requests \