For coreos that’s enough to use something like this

docker run -d \
  -v /home/core/certificates:/etc/nginx/certs:rw \
  --volumes-from nginx-proxy \
  -v /var/run/docker.sock:/var/run/docker.sock:ro \
  jrcs/letsencrypt-nginx-proxy-companion

docker run -d -p 80:80 -p 443:443 \
  --name nginx-proxy \
  -v /home/core/certificates:/etc/nginx/certs:ro \
  -v /etc/nginx/vhost.d \
  -v /usr/share/nginx/html \
  -v /var/run/docker.sock:/tmp/docker.sock:ro \
  -v /home/core/conf.d/external.conf:/etc/nginx/conf.d/external.conf  \
  --restart always \
  jwilder/nginx-proxy

and then not to forget to specify

LETSENCRYPT_HOST=mydomain.com
LETSENCRYPT_EMAIL=my@email.com

in your docker-compose section.

More is here https://hub.docker.com/r/mickaelperrin/docker-letsencrypt-nginx-proxy-companion/