When you instal vlc through apt-get

$ sudo adduser username video
$ sudo usermod -a -G video username

important!, restart your computer or restart your docker container is needed

When you install vlc through snap

root cause

$ snap connections vlc
avahi-control           vlc:avahi-control           -                        -
camera                  vlc:camera                  -                        -
desktop                 vlc:desktop                 :desktop                 -
desktop-legacy          vlc:desktop-legacy          :desktop-legacy          -
dvb                     vlc:dvb                     -                        -
home                    vlc:home                    :home                    -

connect to camera

$ snap connect vlc:camera :camera