Adjust Docker docs with info on DOLT_ROOT_HOST and DOLT_ROOT_PASSWORD

[fulghum]

Changes to root superuser initialization mean that we now create the root superuser as root@localhost (instead of root@%), which means Docker customers need to specify new arguments to start up a Dolt sql-server that can accept connections as root from the host system. DOLT_ROOT_HOST and DOLT_ROOT_PASSWORD are analogues of MySQL's MYSQL_ROOT_HOST and MYSQL_ROOT_PASSWORD environment variables, and work in the same way – users can specify them to override the default host (localhost) and/or default password ("") for the root superuser account.

For example, the following Docker command shows how to start up a dolt/sql-server container where the root account can connect to the container from the host system using the password secret2:

docker run -e DOLT_ROOT_PASSWORD=secret2 -e DOLT_ROOT_HOST='%' -p 3307:3306 dolthub/dolt-sql-server:latest

[zachmu]

LGTM