Make distributed cache to be shared between runners

Description

s3 cache not shared between runners

it's because cache key is identified by runner-id, 162eb344 in this example:

Downloading cache.zip from http://minio-s3:9000/runner/runner/162eb344/project/133/ci-tools 

I have runners configured in docker-compose with REGISTRATION_TOKEN env configured, so scale=3 would create 3 runners but they each have different runner id.

I could hack this and setup CI_SERVER_TOKEN=162eb344....... for each runner, this will work, they will all pull same cache (by 162eb344), but it's definately hack, because all the runners submit under same "runner" in gitlab admin page, whoever they are really 3 different runners.

Proposal

add S3_CACHE_ID or S3_CACHE_DIR variable that all runners can share to identify they wish to share cache.