"Just beware Amazon, Azure and Google – these are Hotel California"
Sorry if this comes across as being picky, but I feel like this is a bit misleading. Azure currently offers both ACS and AKS (in public preview). ACS is a way of deploying a cluster using your orchestrator of choice (Mesos, Kubernetes, etc), whereas AKS is a slightly more managed version that only supports Kubernetes. Either way, you're not locked in - you're running your own Kubernetes cluster that just happens to be in Azure. So if you want to move to another cloud provider or bare metal it's a case of setting up your new cluster and deploying to it.
Obviously this may not be quite as simple as I make out if you use other Azure services (database servers, storage, load balancers, etc), but you're no more locked in than if you were just using Azure to host virtual machines IMO.
I believe AWS and Google also both have similar offerings, although AFAIK the AWS one is in a private preview rather than being publicly available.
So yes, you can be locked in with the public cloud providers, but they're all currently building / have available services that allow you to create a managed Kubernetes cluster, rather than using their own proprietary container services.