Add --auto-register on next Lighthouse update

[michaelsproul]

The validator client should be run with --auto-register, once sigp/lighthouse#1116 is merged and lighthouse-docker is updated.

It should be as simple as adding it here:

lighthouse-docker/scripts/start-validator-client.sh

Lines 14 to 17 in c9daa37

	exec lighthouse \
	--debug-level $DEBUG_LEVEL \
	validator \
	--server http://beacon_node:5052

[paulhauner]

Thank you for updating the docs and thinking of this repo <3