I suppose another option is to run everything in Virtual Machines, then you could run them to any cloud provider you like (or even run your own servers).

What's the reasoning for that requirement, though?

I understand the fear of vendor lock-in, but if AWS decide to put their prices up tomorrow, it's non-trivial to just move everything to Azure (for example). Sure, you have some container images which are portable in theory.. but it's still a large undertaking.

I'd be interested to know if anybody has built with 'cloud agnostic' in mind, and actually needed to migrate to another provider.