If you'd like to run idmapd to map between NFSv4 IDs (e.g. foo@bar.com) and local users, simply provide idmapd.conf to the container.
docker run \
-v /host/path/to/exports.txt:/etc/exports:ro \
-v /host/files:/nfs \
-v /host/path/to/idmapd.conf:/etc/idmapd.conf:ro \
--cap-add SYS_ADMIN \
-p 2049:2049 \
erichough/nfs-server