Transmission container plus volume management

[gpongelli]

This PR is to add Transmission container, no more included as plugin into OpenMediaVault 5 release, that uses OMV shared folders.

To make sure the folders are shared between OMV5 and Transmission container, a volume is needed as explained in approach #2 here. For this reason, it's required the management of new "volumes.yml" file into menu.sh, that can be used by any template, to append all the volume's declaration at the end of docker-compose.yml

Thank you of your awesome job, hope you'll enjoy this PR!

[Slyke]

I have submitted this PR on your behalf over at the fork: SensorsIot#15, If you would like to have your name to the commits, please submit the PR there and let me know so I can remove the PR I created on your behalf.

[gpongelli]

I've just made a PR to your repo .