The adduser command doesn't support UID/GID above 256000 $ docker run --rm -it alpine adduser joecool -u 500000 adduser: number 500000 is not in 0..256000 range I expect it to handle the maximum of an 2,147,483.646 (unsigned int) on 64 bit platforms (which the above alpine image is).
Increase CONFIG_LAST_ID in .your .config. Testing with CONFIG_LAST_ID=1999999999: # ./busybox adduser joecool -u 999999999 Changing password for user joecool. New password: BAD PASSWORD: The password is a palindrome Retype new password: passwd: all authentication tokens updated successfully.