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.