rails db:system:change should update the Dockerfile too

[dhh]

If you start a new Rails app with the default sqlite db adapter, you can change the config/database.yml and Gemfile declarations to mysql using rails db:system:change --to=mysql. But this doesn't update our new Dockerfile to exchange sqlite to default-mysql-client for the deployment section and adding default-libmysqlclient-dev to the build section. It should.