This is not currently possible. I may have a different view than others, but I’ve always seen containers as being at their best when they are expendable. For example, an app server that deploys from a git repo and can be spun up on demand, then removed without harm. Perhaps it pulls content from object storage, allowing you to only pay to store the content when no app servers are deployed. Obviously use cases vary.
I also want to note that this is not possible with other droplets, so that I make sure you are not billed anything that you were not intending to be billed. While you can power a droplet off, this does not halt billing as we continue to reserve the physical compute resources, allowing you to power on the droplet at any time without having to migrate it to available hardware. However, you can reduce the billing of a droplet by taking a snapshot and then destroying the droplet. This is billed at $0.05 per GB, per month, for the snapshot.