1. go-smtp is replaced by a fork that reverts StartTLS removal.
2. SASL LOGIN is no longer supported by upstream go-sasl, readded disabled by default.
3. Updated endpoint code to match new go-smtp authentication interfaces.
4. certmagic repo had some renames
5. Minimum Go version increased to 1.23 to match dependencies.
* Use TLS filenames same as certbot (see #350).
* Put the Docker-specific maddy.conf in the repo (see #350).
* Set OCI labels for the image in CI
* Move Docker-specific documentation from Docker Hub into docs/
* Add .dockerignore
hub.docker.com image for 0.4.2 was built using 1.15.4 but
the corresponding change was not commited to the repository.
This is important to get the fix for CVE-2020-28362 (DoS vuln. in TLS
client).
