Fix msmtprc in Docker Hub image

[Flightkick]

I forgot about msmtprc while porting the application to a Docker Hub image so e-mail support is not on par with the other set-ups. The fix would include copying the docker-php/msmtprc file to the image. Either use environment variables directly in that file if that's supported or make the init.sh file copy the config and replace the placeholder values by environment variables. This would also include installing msmtprc in the container image (apt).

I'm currently not able to spend time on it. Here is a reference for later (not 1:1 applicable): https://owendavies.net/articles/setting-up-msmtp/