The ALB reaches backends over the private VPC network, so the Nginx
instances no longer need public IPs. Drop associate_public_ip_address
and map_public_ip_on_launch, and swap the instance public-IP outputs
for private-IP ones.
The *.elb.spinifex.local DNS name does not resolve from the operator's
host, so document an AWS CLI one-liner (describe-load-balancers with
an AvailabilityZones[].LoadBalancerAddresses[].IpAddress query) that
retrieves the ALB's public IP for curl/browser testing.
Josh Siviour