issues with high-numbered UIDs/GIDs inside inner container

# 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
```