Add example of nginx+step-ca

examples/docker/Makefile

examples/docker/ca/Dockerfile

examples/docker/ca/pki/config/ca.json

examples/docker/ca/pki/secrets/intermediate_ca.crt

examples/docker/ca/pki/secrets/intermediate_ca_key

examples/docker/ca/pki/secrets/root_ca.crt

examples/docker/ca/pki/secrets/root_ca_key

examples/docker/docker-compose.yml

examples/docker/nginx/Dockerfile

examples/docker/nginx/certwatch.sh

examples/docker/nginx/entrypoint.sh

examples/docker/nginx/site.conf

examples/docker/password.txt

examples/docker/renewer/Dockerfile

examples/docker/renewer/crontab

examples/docker/renewer/entrypoint.sh