[basics] Add instructions for using MySQL from a Docker container

[jgbarah]

Following advice by @filmaj in #36.

[filmaj]

Nice, thanks!

If you can tweak which mysql docker image to use, as per my suggestion, then you can remove the last paragraph as well (and make the instructions simpler).

Thanks for putting this together!

[filmaj]

Suggested change

	docker run --env MYSQL_ALLOW_EMPTY_PASSWORD=1 --env MYSQL_ROOT_PASSWORD="" -p 3306:3306 -d mysql/mysql-server:latest
	docker run -d -p 3306:3306 -e MYSQL_ALLOW_EMPTY_PASSWORD=yes mysql

With the above suggested change, you can also remove everything about creating a new root user, connecting to the running docker, etc - much simpler! Here are the two docker commands I use to spin up the various infrastructural bits for running tests for grimoirelab programs:

docker run -d -p 3306:3306 -e MYSQL_ALLOW_EMPTY_PASSWORD=yes mysql
docker run -d -p 9200:9200 -p 5601:5601 nshou/elasticsearch-kibana