Update docs for image variable in custom executor
What does this MR do?
Update docs for image variable in custom executor
Why was this MR needed?
In !1666 (merged) we
introduced a new variable CUSTOM_ENV_CI_JOB_IMAGE
which exposes the
image
to the custom executor.
In !1813 (merged) we introduced it to every executor and part of the predefined varaibles dc93cad6