I would use them just to save the whole setup/install thing.
When sql releases were every 2-3 years you would upgrade the hardware at the same time, annually/semi-annually? Less likely.
Being able to change up a version within minutes is a significant improvement over the whole msi install thing even if you did script it.
I haven't yet seen a production problem that Docker containers solve better than IaaS VMs (AWS EC2, Azure VMs, Google Compute Engine.)
I've seen a lot of development problems that containers help solve, like quickly building out Development Edition VMs for continuous integration testing. Since Dev Edition is free, rapidly building/destroying containers makes sense. However...
> dynamically adding Always On Availability Group read-only replicas
Readable AG replicas require SQL Server Enterprise Edition, which is $7k USD per core. That pricing tends to dwarf the hardware it's running on, so it usually makes more sense to run bare metal on these if it's a long-term scale requirement. (I have indeed done temporary scale-out reads on VMs, it's just usually for things like Black Friday workloads for short periods of time.)
(And then at the end, I went to add a disclaimer about what I do and why I'm answering, and then I went, "Oh, wow, it's James Anderson asking this." OK, so we both know each other, but here's my answer anyway for the rest of the HN community.) So source: Microsoft SQL Server consultant.