Fix k8s entrypoint management for build container

Summary

Steps to reproduce

This issue is to make sure that the changes made in the MRs below are re introduced in %17.0

• Add custom entrypoint for the build container f... (!4394 - merged) • Baptiste Lalanne • 16.6
• Sanitize image entrypoint to remove empty string (!4452 - merged) • Romuald Atchadé - OOO until Jan 11th, 2026 • 16.6

Those MRs changes the behaviour of the executorkubernetes WRT to build container entrypoint and were thus reverted from the %16.6.

They will be re introduced as breaking change in %17.0

A documentation entry should also be added to warn users in advance.