Scylla Docker: Add a config file option

[tzach]

Installation details
Scylla version (or git commit hash): 3.0

Cassandra Docker allows using an external config file using run ... cassandra -Dcassandra.config=/path/to/cassandra.yaml
Source: https://docs.docker.com/samples/library/cassandra/#configuring-cassandra

It will be useful to have a similar function for Scylla Docker

[tzach]

This is possible via mount

$ docker run --name some-scylla --volume ~/master_scylla.yaml:/etc/scylla/scylla.yaml -d scylladb/scylla