doc: how to switch dashmate dash core version

[coolaj86]

This didn't work for me just now, so I'm deleting the script associated with it, but I wanted to have it here just in case it comes in handy later:

Update: It's required to either have set the default config, or to use the --config=<name> option.

dashmate config default local_seed

dashmate config set--config local_seed core.docker.image dashpay/dashd:20.1.1
dashmate config set--config local_1 core.docker.image dashpay/dashd:20.1.1
dashmate config set--config local_2 core.docker.image dashpay/dashd:20.1.1
dashmate config set--config local_3 core.docker.image dashpay/dashd:20.1.1

Note: set is a third-level subcommand, so the --config local_1 option must come AFTER that.