Add Mail-Hog to docker-compose stack

[nhoening]

When the docker compose stack starts up, it has much functionality, but resetting the password of the toy-user breaks (500).

We can add the Mail-Hog image to let the emails work. This is how any dev can add it:

docker run -p 8025:8025 -p 1025:1025 --name mailhog mailhog/mailhog
export MAIL_PORT=1025  # better to set in your local flexmeasures.cfg, no credentials necessary

Now emails (e.g. password-reset) are being sent via this local server. Go to http://0.0.0.0:8025/ to see the (or is localhost better in this case?)

Adding a mailhog/mailhog image to docker compose is helping that stack to work better.

Tasks:

1. Confirm the problem
2. Adding the mailhog image (add a comment that http://0.0.0.0:8025/ is where the mails go)
3. Setting the config setting: export MAIL_PORT=1025
4. Confirming resetting the password is not breaking now.
5. Add a comment to the docker-compose part of the docs about http://0.0.0.0:8025/, as well.