When creating an Azure Container Instances using the portal, I discovered a UI bug with the environment variable textbox.  I was trying to set one with a storage key but the portal refused it displaying an error message.

Strange!  To be sure, I then tried to create one using the Azure CLI and it worked.

az container create --resource-group myResourceGroup /
--name mycontainer --image microsoft/dotnet /
--dns-name-label aci-guybtest --ports 80 /
--environment-variables key="abc123+IWq6MeY0uQoRSMMhkrnG1oBhwA=="

I notified Microsoft and it turns out that the portal UI is overzealous and this will be fixed in the near future.