azure: Make sure docker directory exists

Looks like the Azure image changed recently so let's account for that!

.azure-pipelines/steps/run.yml

@@ -50,6 +50,7 @@ steps:
 # on since libstd tests require it
 - bash: |
     set -e
+    sudo mkdir -p /etc/docker
     echo '{"ipv6":true,"fixed-cidr-v6":"fd9a:8454:6789:13f7::/64"}' | sudo tee /etc/docker/daemon.json
     sudo service docker restart
   displayName: Enable IPv6