This hook is called from porter build prune for each region
before any regions are pruned
(as opposed to before each region is pruned)
Standard and Custom environment variables
PORTER_ENVIRONMENT
AWS_REGION
AWS_DEFAULT_REGION AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY
AWS_SESSION_TOKEN AWS_SECURITY_TOKEN are available to enable AWS SDKs to
make API calls and are the credentials of the assumed role.
AWS_CLOUDFORMATION_STACKID is known after provisioning and is set by porter.
AWS_ELASTICLOADBALANCING_LOADBALANCER_DNS is the DNS of the provisioned ELB
(which may be an empty string if the ELB is internal). This is not the DNS of
the ELB that instances were promoted into.