Split `make docker` for GitLab Runner Helper
GitLab Runner helper is built using `make docker` which builds the binary, creates a Docker container and extract the files system of that container for publishing at a later stage. Because of #3755 we have to split this command up so we generate the binaries and the docker archive separately since Windows containers cannot be built on Linux systems or vice versa. We are keeping the binary generation in one command to have both Windows and Linux binaries consistent, and built them on a Linux platform where `make` is installed. The make command already sets up the $GOPATH, dependencies and environment for building the binaries so there is no point of duplicating that. Then diverge when building the docker containers, since they have to be done on different platforms. Windows does have GNUMake http://gnuwin32.sourceforge.net/packages/make.htm but it's fairly outdated and doesn't seem to work when installing it on Windows server 2019. Drop support for `USE_PRECOMPILED_IMAGES` since it's just extra complexity that is not needed, and it doesn't seem to be used anywhere. If the user runs `make helper-docker` and the binaries are already present they are not rebuild.
Showing with 66 additions and 40 deletions