Add --dry-run support to the DNSSEC signer

Fixes issue #1.
4 jobs for master in 4 seconds
Status Job ID Name Coverage
  Test
failed #10810
run_tests

00:00:04

 
  Build Src
skipped #10811
build:src
 
  Build Pkg
skipped #10812
build:pkg
 
  Upload
skipped #10813
upload:pkg
 
Name Stage Failure
failed
run_tests Test
Running with gitlab-runner 12.1.0 (de7731dd)
on gitlab-ci-shared 1fc9a9d2
Using Docker executor with image ai/test:base ...
Pulling docker image ai/test:base ...
ERROR: Job failed: Error response from daemon: pull access denied for ai/test, repository does not exist or may require 'docker login': denied: requested access to the resource is denied (executor_docker.go:192:1s)