Add permissions on HV transcoding devices in docker container

[m-larin]

Gerbera start in docker from unpermitted user. When call ffmpeg with HW transcoding options rase error about the unavailability of the device. This commit fixes the error.

[KarlStraussberger]

Thanks for that!

I'm not sure whether this end up being a security issue, because you can use the container to bypass more restrictive permissions on the host system.

[KarlStraussberger]

Instead we might add gerbera user to the group with access to the devices.

[m-larin]

Redone according to your comments. Now the gerbera user is added to the video group, and this group is given rights to the device video

[KarlStraussberger]

Thank you for your contribution.