fix(logger): remove logger-build container after "docker cp"

This works around docker/docker#8632 and an error in the Makefile
evaluation order logic.
Merge pull request #2190 from mboersma/fix-docker130-builds

fix(logger): remove logger-build container after "docker cp"
ref(user-data): "make discovery-url" copies from user-data.example

This also adds contrib/coreos/user-data to .gitignore, so that local
changes to this essential file do not cause "git status" to mark the
repo as dirty.