Create load service docker-compose file

This addition makes it easy to start into a load scenario right
away with docker-compose, by referencing 2 docker-compose files.

By default, only the file with the services will be loaded, for
starting the load-gen right away, use
`docker-compose -f docker-compose.yaml -f docker-compose.load.yaml OPERATION`

The given config will use the internal container network to
produce some load.

docker-compose.load.yaml

@@ -0,0 +1,10 @@
+version: '3'
+services:
+  load:
+    image: ${REPO}/rs-load:${TAG}
+    environment:
+      HOST: http://web:8080
+    networks:
+      - robot-shop
+    depends_on:
+      - web