Allow execute entrypoint script in docker container

[elbandi]

Describe your core improvement

It would be good, if we can easy add custom script to container image, and the entrypoint script will execute it at start.

This is a good sample:
https://github.com/nginx/docker-nginx/blob/e4d5453581d9d3618f77c4aeccf2e6171a1cd6ff/mainline/debian/docker-entrypoint.sh#L13-L45

Current limitations

Cannot run script on startup in docker

Technical benefits

Additional context

No response