I strongly disagree with this.
Pet usually means there's a whole lot going on the server (or the container, for that matter), and you need more tools in order to maintain whatever that is. PostgreSQL for example. In order to run it "properly", you need to manually set up replication, backups, failover scenarios, monitoring, logging, etc,... And most containers are kept simple so they only run one process (or very few of them) - and they often come with very little extra libraries installed, which makes any kind of advanced setup much harder.
Pet containers are stateful, and that state is often data. Data that is crucial to your business. I wouldn't trust a container to handle it, it's just too dumb (by design, I guess?)
