Previous versions of Docker image runs scylla as root, but cb19048
accidently modified it to scylla user.
To keep compatibility we need to revert this to root.
Fixes#10261Closes#10325
Bring supervisor support from dist/docker to install.sh, make it
installable from relocatable package.
This enables to use supervisor with nonroot / offline environment,
and also make relocatable package able to run in Docker environment.
Related #8849Closes#8918