Add support for the armv7/armv7l/aarch64 architecture

Even though we now can run the standalone gitlab-runner binary under armv7/armv7l/aarch64 (e.g. with https://gitlab-runner-downloads.s3.amazonaws.com/latest/deb/gitlab-runner_armhf.deb), this chart does not yet support that use-case, as such, it would be really useful to be able to run this chart under a raspberry pi cluster.

This chart is not working because the used containers available at https://hub.docker.com/r/gitlab/gitlab-runner/tags?page=1 are only available in the linux/amd64 arch.