Add run_id as env variable to base image.

Add an environment variable to our base image and set the MHub build-pipeline `run_id` at build time such that we can tie back our containers to specific builds (and all connected release and versioning information) via the run_id.

We may consider renaming `run_id` to `build_id` or `mhub_build_id` to prevent confusion with docker-related build IDs.