Expose image:entrypoint to custom executor
Description
Following the implementation of #4357 (closed) it would be nice to expose the image:entrypoint to the custom executor scripts with a variable like CUSTOM_ENV_CI_JOB_IMAGE_ENTRYPOINT. Custom executors like podman would benefit a lot from this feature as many container images (like the official terraform image which has /bin/terraform in entrypoint) does not have a "compatible" entrypoint.
Now, we have to build an intermediate image (which only overwrites the entrypoint of the original image) in order to overcome this missing information inside the custom executor scripts.
Proposal
Expose image:entrypoint to custom executors with ENV variable (for example CUSTOM_ENV_CI_JOB_IMAGE_ENTRYPOINT)
Links to related issues and merge requests / references
A similar implementation for image:name is done in #4357 (closed)