Update mysql image in WordPress Docker setup to work on arm architectures

[justinahinon]

When following the steps in ​https://github.com/WordPress/wordpress-develop/blob/master/README.md to set up a WordPress Core development environment, you'll run into an error at this step: npm run env:start when the script is pulling the mysql image.

ERROR: no matching manifest for linux/arm64/v8 in the manifest list entries

This is due to mysql not supporting yet arm architectures (per ​https://github.com/docker-library/mysql/issues/318#issuecomment-339460074).

This PR allows pulling MySQL image on arm architectures while

Fixes https://core.trac.wordpress.org/ticket/52591

[desrosj]

I was working through this with @afragen over the weekend as well. It's likely that this will be fixed upstream, so I'm hesitant to make this temporary change.

For the time being, you can easily work around this with two approaches.

1. Create a docker-compose-override.yml file and define the mysql container with the platform specified.
2. Use mariadb instead of mysql. You can do this by changing the .env file LOCAL_DB_TYPE and variables locally to mariadb and latest or by exporting those variables using command line before running npm run env:start.

I am writing a Build/Test Tools post for the Make Core blog detailing several tooling updates and I will include a small section detailing this and other small M1 issues that have been discovered.