Support passing max-email-attachment-size parameter through docker-compose.yml file

[hostingnuggets]

The gvmd Docker container image has no way to pass the following parameters to gvmd as env variable in the docker-compose.yml file:

  --max-email-attachment-size=<number>          Maximum size of alert email attachments, in bytes.
  --max-email-include-size=<number>             Maximum size of inlined content in alert emails, in bytes.
  --max-email-message-size=<number>             Maximum size of user-defined message text in alert emails, in bytes.

So right now one has to manually edit /lib/systemd/system/gvmd.service service file inside the container and restart it. Then when the Docker container is upgraded these changes are lost...

This would be very practical to avoid the following warning when attaching PDF files to mail report because by default it is limited to 1MB which is almost never enough:

Note: This report exceeds the maximum length of 1048576 characters and thus was truncated.

Maybe it would also be a good idea to change the default to 10MB.