Honor entrypoint for build and helper images with exec passthrough

What does this MR do?

Allows GitLab Runner to both honor entry point and also function with exec passthrough (exec "$@")

This also works with FF_USE_LEGACY_KUBERNETES_EXECUTION_STRATEGY: true

Why was this MR needed?

Containers with an entrypoint with exec passthrough were not working.

What's the best way to test this MR?

Try to run a container such as Consul image which has exec passthrough https://github.com/hashicorp/docker-consul/blob/5607edeffa1c759ce996b437374be9e6613bdc15/0.X/docker-entrypoint.sh#L114

What are the relevant issue numbers?

#4125 (closed)

#4125 (comment 740372996)

#28484 !3095 (merged)

close #28807 (closed)