app-emulation/docker: insecure-registry compatibility

Ick, can we recommend that people use a dropin like we did here please? https://coreos.com/blog/docker-1-3-2-security-update/

I want to stick with a consistent way to modify units, the only reason why /run/docker_opts.env was added instead of using a dropin was for the awkward two-stage docker setup. I would rather not extend that pattern.