Create a i386 docker container running caddy.

Uses the official caddy builder project https://github.com/abiosoft/caddy-docker/blob/master/BUILDER.md to build an i386 compatible version of the caddy server http://caddyserver.com.

Note that by using this image you are agreeing to the Let's Encrypt Subsciber agreement at https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf