Added NGINX_WORKERS configuration
Don't show server_tokens

Showing server tokens reveals the nginx version number to would-be attackers. The version number is an easy way to figure out security exploits.
Make target file name explicit
Include `STACK` environment variable in file name
Vendor binary for "cedar-14" stack
Set the stack to cedar

Because this does not appear to be set on dokku's docker image.
Include video modules when building nginx