issues with high-numbered UIDs/GIDs inside inner container

[johnstcn]

Problem

If you try to add a user inside the inner container with a UID above 65535 and attempt to su to that user, you will get the error cannot set groups: invalid argument.

We should document a process users can follow to enable the usage of such UIDs/GIDs inside the inner container.

Details

Inside the envbox container, we define a sub-UID/GID range 100000-165535 for the coder user (note: sysbox-mgr automatically adds a separate UID range for the sysbox user):

# cat /etc/subuid
coder:100000:65536
sysbox:165536:65536 # only appears after sysbox is started
# cat /etc/subgid
coder:100000:65536
sysbox:165536:65536 # only appears after sysbox is started