Change sandbox user UID from 1000 to 1000660000 in custom image examples and E2E tests.
Using a high UID range (1000000000+) prevents conflicts with host users when running
without user namespace remapping, where container UIDs map directly to host UIDs.
This resolves fork failures caused by RLIMIT_NPROC enforcement when the host user
already has many threads running.
Signed-off-by: Derek Carr <decarr@redhat.com>