Installs ca-certificates, required for the "release message" feature.
With the current [mailhog/mailhog](https://hub.docker.com/r/mailhog/mailhog/)
docker image, releasing messages to an SMTP server via TLS fails:
> Failed to release message: x509: failed to load system roots and no roots
> provided
Removes any build dependencies after install, optimising docker image size.
The image size is about the same as the Rocker based build, but works with plain
Docker and can therefore be used via Docker Hub Automated Builds.
Runs with a non-privileged user account instead of as root, increasing security.
Assigning the user ID 1000 allows host volume mounts (e.g. for maildir storage)
to work seamlessly with docker-machine.
Installs MailHog in `/usr/local/bin`, which is in the system `PATH` and allows
to use a simplified entrypoint (`MailHog`).
