Still no user namespace support? I don't get how one can use for production websites without this. Especially if your run arbitrary containers from the docker registry. Or this not the suggested model anymore?
To answer your question, no, you don't need to map all the users inside the container to users on the host.
pflask user namespace support is quite limited right now: with the --user option you tell pflask to map the outside user that is running pflask, to the inside user specified by the option. Let's say you run something like:
$ sudo pflask --user=some_user ...
The whole point of this feature was the possibility of running pflask as non-root, so you could map a normal user on the host to the root user inside the container and still be able to call mount() (although there are several limitations), so it's only possible to map one user right now, however it shouldn't be difficult to add another option to map additional users (feel free to open a GitHub issue if you need this).
Integration of user namespaces into the developer-facing tools is ongoing, but there is an design discussion to finish on how to best handling uid mapping without breaking shared volumes.
TLDR: if you want to customize your docker install to support user namespaces today, you should try runC.