Add msmtp log to docker-compose volumes

[Muchaszewski]

Hi,

I had some issues with figuring out why my emails were not working correctly.

If by default in docker-compose /var/log/msmtp.log was exposed that would save me a lot of time.

Porposal:
Add /var/log/ folder or msmtp log file to docker-copose volumes

[jjethwa]

Sounds like a great idea! Would you like to submit a PR?