What’s in the docker-entrypoint.sh script? This is probably the right path, except that you can’t RUN the script during the docker build phase, you need to run it as an ENTRYPOINT (and make sure to also run the base mysql container’s entrypoint script as well).
Speaking of the stock mysql image’s entrypoint script, you should read through https://github.com/docker-library/mysql/blob/master/5.6/docker-entrypoint.sh, which makes it sound like all you need to do is COPY the one file to /docker-entrypoint-initdb.d/; you don’t need to provide or replace your own entrypoint script.
Remember that these details are visible to anybody who runs docker history on your image, and baking passwords like this into an image is distinctly not a best practice.