Add feature flag to use old API for helper
With gitlab-org/gitlab-runner!1195 & gitlab-org/gitlab-runner!1201 the bash scripts were moved to Go so that they can be used for windows containers. It's safe to use these commands on new helper images, which is automatically downloaded when the user does not specify a `helper_image` in the config file. If the user has specified a helper image which does not use the new API will be considered as a breaking change. If the `helper_image` is specified and the newly introduced feature flag `FF_DOCKER_HELPER_IMAGE_V2` is not set, use the old command when starting the container.
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