Make docker executor fail if a service alias is not DNS-1123 compliant
In #2229 (closed) we implemented service aliases in the form of Kubernetes host aliases. Kubernetes, however doesn't allow host aliases that are not
DNS-1123 compliant. In the k8s executor we fail the job if an invalid alias is supplied. The docker executor does not. We should probably bring the docker executor in line at some point. As @steveazz we might not want to do this for
13.0 but rather at a later stage.