User and group ID mappings

[LaurensRietveld]

Are you open to a PR that enables user and group ID mappings? Considering that docker has no built-in solution for this (moby/moby#7198), the files in the shared volume end up as owned by the container mongodb user.
A simple script in the entrypoint that modifies the mongodb user and group ID (based on two environment variables) would solve this issue.

[tianon]

I think the approach we've taken in docker-library/elasticsearch#77 is probably the one we want to follow here too (see especially the additional notes in docker-library/elasticsearch#14 (comment) about why that particular solution is nice).

[yosifkit]

#81 fixed this.