Expose image to custom executor
Description
With #2885 (closed) we added the MVC for the generic executor, which allows the user to setup their own environment.
We have the image keyword which is usually used to define what Docker image to be used. But this can be easily used for podman or lxc, we need to expose the value of image as an environment variable to jobs executed by the custom executor so that they are able to pull the right image.
Proposal
Send the value of image as an environment variable CUSTOM_ENV_CI_JOB_IMAGE or something similar.