Document extending the runner environment

What would you like added?

I would like the preferred solution for extending the runner environment/image and runner PATH to be documented in https://github.com/actions/actions-runner-controller/blob/master/docs/about-arc.md#software-installed-in-the-runner-image

I am currently setting PATH via /runnertmp/.env after reading https://github.com/actions/actions-runner-controller/issues/1135. I tried the .path solution mentioned and it did not work.

I would make a PR but am not sure if the .env solution is the preferred/correct solution for adding binaries to the runners path and would like an official stance

Why is this needed?

Users who need custom paths in their runners do not have a clear path to integrating them into the runners image at this time.

I needed to add gcloud to the runner image to enable docker gcr authentication for the job container option and the override of PATH in my image was being ignored.

Previously, I used summerwinds/action-runner:v2.285.1-ubuntu-20.04 as my base image, added the gcloud binaries and updated my PATH. When I upgraded to newer runner versions, my changes to the PATH were being ignored.

Additional context

I spent significant time reading the docs and could not find a solution. Reading through https://github.com/actions/actions-runner-controller/issues/1135 required me to try a variety of solutions and did not explicitly state which worked/was preferred

Start with docs/about-arc.md at “Software installed in the runner image” and read issue #1135 for the existing .path and /runnertmp/.env approaches. Verify which method works for extending the runner image and PATH, then document the preferred solution and a clear completion example for custom binaries such as gcloud.