Change mysqld to output to stderr, fixes #1136

[rfay]

The Problem/Issue/Bug:

OP #1136 - it was really hard to figure out what would go wrong with mysqld when something was wrong. You had to capture the container, then docker exec into it, then view the logs.

How this PR Solves The Problem:

• mysqld logs now land in stderr, which means they're captured by ddev logs -s db
• Update docs that incorrectly stated that the logs were captured in files inside the container.

Manual Testing Instructions:

• Start a project
• ddev logs -s db to view the full startup of the container.

For extra credit, break the thing:

• ddev ssh -s db
• rm /var/lib/mysql/mysql/*.MYD
• ddev rm && ddev start. It won't come up.
• ddev logs -s db to find out what's wrong.

Automated Testing Overview:

No tests were added.

Related Issue Link(s):

OP #1136

[andrewfrench]

ddev logs -s db shows detailed mysql logs, especially useful when the db service is broken. Works as expected 👍