Add job variables back to the build container
## Problem
The issue https://gitlab.com/gitlab-org/step-runner/-/issues/284+ introduced a change that removed environment variables from the build container running in a job. This is thought to be breaking change for those who set the entrypoint when the docker container runs.
## Proposal
Find a way to revert the change so that runner can be released with steps without going through the breaking change process.
## Implementation suggestion
Use an environment variable to store the names of the job variables. The step-runner will use the variable to know which variables are environment variables and which are job variables.
issue