autoset --registry-mirror flag if user's docker daemon has been configured with a mirror

[medyagh]

if user's docker has been configured with a registry-mirror we should detect that and auto set --registry-mirror flag

example docker system info

Client:
 Debug Mode: false

Server:
 Containers: 6
  Running: 1
  Paused: 0
  Stopped: 5
 Images: 9
 Server Version: 19.03.13
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 8fba4e9a7d01810a393d5d25a3621dc101981175
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.4.49+
 Operating System: Debian GNU/Linux 10 (buster) (containerized)
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 15.64GiB
 Name: cs-719547079457-default-boost-db594
 ID: OL2H:JX4W:5IFS:3BOF:7B2S:5UBR:MT65:346V:UUFU:TZSD:F57F:RGYW
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Username: chiwoojo
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Registry Mirrors:
  https://us-mirror.gcr.io/
 Live Restore Enabled: false

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