Detects the host architecture early in the preflight sequence. On any
architecture other than x86_64/amd64, prints a 5-line warning that
NOMAD officially supports x86_64 only, points at PR #419, and sleeps
10 seconds before continuing. Ctrl+C aborts cleanly before any Docker
work happens.
Preserves the community/hacker path: ARM64 users running with QEMU
binfmt_misc emulation can still let the install proceed. The change
just stops the silent 2.7GB amd64 pull on architectures where it
will not work, which leaves partial images and /opt/project-nomad/
debris that confuse first-time users.
Reported in #782.
cb129d2713