Move deploy procedure away from deprecated gitlab-runner exec

gitlab-runner exec is unfortunately deprecated as of gitlab runner 10.0 and is scheduled for removal for gitlab 11.0 (I think). Ref: https://gitlab.com/gitlab-org/gitlab-runner/issues/2710

As noted here the official deploy process seems to depend on this functionality: https://f-droid.org/docs/Deploying_the_Website/

An alternative has to be found.