docker container as non root user

[mergwyn]

The docker container runs as root by default, and this results in a warning Don't run this as Root!. However if you set a different user id using user: 3001:3001 in the compose file then you get an error open /tmp/xteve/permission.test: permission denied (Specified temp folder path is invalid, fallback to /tmp) - EC: 1015

Steps to reproduce the behavior:
set user: to a non root value in the compose file

Expected behavior
The tmp directory should be created with the correct ownership