Add ability to set the MTU size of the docker in docker container (#385)

* adding abilitiy to set docker in docker MTU size

* safeguards to only set MTU env var if it is set

runner/startup.sh

@@ -33,5 +33,9 @@ for process in "${processes[@]}"; do
     fi
 done
 
+if [ -n "${MTU}" ]; then
+  ifconfig docker0 mtu ${MTU} up
+fi
+
 # Wait processes to be running
 entrypoint.sh