Make Docker Executor ARM compatible

During our testing of new Omnibus auto-scale infrastracture we were hit by a problem that GitLab Runner Docker executor doesn't work on ARM.

This makes it possible, by:

1. Prepare separate prebuilt image for ARM (it inflates binary by 15MB). The image is based on scaleway/alpine,
2. This also simplifies our image handling: we now build two images that are flattened which are then docker import'ed to Docker Engine (possibly this will solve the no-image errors that we see on external packages),
3. The single image contains all helper scripts (see /dockerfiles/build)

This requires to have apt-get install binfmt-support qemu-user-static installed on builder machine.

@tmaczukin What do you think?

Download the standalone binaries

• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-linux-386
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-linux-amd64
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-linux-arm
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-darwin-386
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-darwin-amd64
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-windows-386.exe
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-windows-amd64.exe
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-freebsd-386
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-freebsd-amd64
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/binaries/gitlab-ci-multi-runner-freebsd-arm

Download one of the packages for Debian or Ubuntu

• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/deb/gitlab-ci-multi-runner_i386.deb
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/deb/gitlab-ci-multi-runner_amd64.deb
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/deb/gitlab-ci-multi-runner_arm.deb
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/deb/gitlab-ci-multi-runner_armhf.deb

Download one of the packages for RedHat or CentOS

• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/rpm/gitlab-ci-multi-runner_i686.rpm
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/rpm/gitlab-ci-multi-runner_amd64.rpm
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/rpm/gitlab-ci-multi-runner_arm.rpm
• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/provide-arm-image/rpm/gitlab-ci-multi-runner_armhf.rpm

cc @marin

Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/364