helper images should not be bundled as binary data inside gitlab-runner executable

Description

Today we are cross-compiling gitlab-runner-helper, building a docker image, exporting it as an archive and the bundling such archive as binary data inside the runner itself.

This feature has a huge impact on build times and do not allow to go get gitlab-runner.

Furthermore supporting new architectures increase both build times and the final binary size.

Proposal

We should build and publish helper images online and use them.

For those who run gitlab-runner without access to dockerhub we should change docker.getPrebuiltImage allowing to load images from the hard disc.

Next iteration

  • Increase security signing local images

Links to related issues and merge requests / references

/cc @ayufan