There's a need to activate the devtool-6 SCL before running the yarn install command, to have node-gyp correctly compiling the needed binaries.

Changing the link  for ffmpeg toward a tutorial with an actual mirror of ffmpeg, and with less hassle.
Adding the deendencies to devtoolset-6 and centos-release-scl to provide a C++11 compatible G++

* Add permissions instructions for the data volume

* Remove useless newline

* Run the installation as the peertube user

* Run the installation as the peertube user

* Specify Redis major version