Python's mysqlclient requires libmysqlclient-dev libraries to build and install. This image uses a 2-stage Dockerfile to build a mysqlclient wheel, so the final image does not require the libmysqlclient-dev libraries or build toolchain.
For comparison, the first stage with the dev dependencies produces a 411MB image. The second stage, where the mysqlclient wheel is installed, is only 149MB.