Add arm64 Docker images for gitlab/gitlab-runner

What does this MR do?

This MR adds ARM64 Alpine/Ubuntu Docker images and makes images multi-platform. I've pushed an example to my Docker Hub space: pombeirp/gitlab-runner .

A big shout out to @solidnerd for his work on !725 (closed)!

Once this MR is merged, a follow-up PR to add support for 32-bit ARM should be very straightforward. It would also be nice to have the gitlab-runner-helper image be published as a multi-platform image, rather than relying on tags.

Relevant documentation:

• https://github.com/docker/buildx
• https://www.docker.com/blog/getting-started-with-docker-for-arm-on-linux/
• https://docs.docker.com/buildx/working-with-buildx/

Why was this MR needed?

There is a desire to have the GitLab Runner Docker image run on ARM64 platforms. See #2076 (closed).

Testing

Manual QA can be found 👉 !1861 (comment 328302496)

Are there points in the code the reviewer needs to double check?

• This MR removes the release tag since that build agent doesn't have a recent Linux kernel required by this MR.
• This MR leverages the new docker buildx command which is experimental (added in a separate MR). The previous way of building multi-platform images involved a community tool called manifest-tool.
• When docker buildx builds multi-platform images, those images cannot be loaded into the local image registry, they must be pushed directly to a remote registry. Therefore we'll need to tag and push the images directly when building, requiring some changes in the ci/release_docker_images script.
• In order to use docker buildx successfully, we need to create:
  • a docker context (which contains the location of the host and certificate information)
  • a build node that uses the docker-container driver rather than the default docker driver.
  • a host containing a Linux kernel with version >= 4.8
  • binfmt_misc correctly installed on the host and with handlers for AMR64 binaries (we do this in .gitlab/ci/release.gitlab-ci.yml with docker run --rm --privileged multiarch/qemu-user-static --reset -p yes)
• When publishing, we use the newer buildx logic in the xbuild_and_publish function. If building locally, we fallback to the existing logic (because buildx doesn't support adding incompatible platforms to the docker images list)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug - N/A
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #2076 (closed)