This change adds a SCYLLA_REPO_URL argument to Dockerfile, which defines
the RPM repository used to install Scylla from.
When building a new Docker image, users can specify the argument by
passing the --build-arg SCYLLA_REPO_URL=<url> option to the docker build
command. If the argument is not specified, the same RPM repository is
used as before, retaining the old default behavior.
We intend to use this in release engineering infrastructure to specify
RPM repositories for nightly builds of release branches (for example,
3.1.x), which are currently only using the stable RPMs.
4a9b2a8d96