Build linux/riscv64 platform for runner image

Ludovic Henryrequested to merge
luhenry/base-images:riscv64-gitlab-runner-image into main

Adds the linux/riscv64 base images for runner on platforms ubuntu and alpine. These base images are to be used in https://gitlab.com/gitlab-org/gitlab-runner.

It currently depends on https://github.com/Yelp/dumb-init/pull/305 for dumb-init to be available on riscv64

Edited by Axel von Bertoldi

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