I pray to god nobody uses docker and postgres or any other database until this question is resolved. The most rational explanation for why postgres would be 'faster' under docker virtualization than it is raw-on-the-operating-system would be that libcontainer/docker are skipping over some fundamental atomicity guarantee.
Docker is just glue code. That's all it is. It sets up your bind mounts, cgroups and namespaces. It then execs the process and gtfos. The way the kernel is dealing with the namespace'd postgres is the difference.
I'd say it's much more likely the pre-configured docker digital ocean makes available is on hardware that isn't as heavily used as a regular old postgress configuration.
That is, there's more load in general on the postgress machine rather than the docker machine, you don't get perfect isolation in those environments.
