Add Docker registry mirror to Docker daemon on systemd (Ubuntu 16.04)

What Docker registry is

The Registry is a stateless, highly scalable server side application that stores and lets you distribute Docker images. The Registry is open-source, under the permissive Apache license.

Why use it

You should use the Registry if you want to:

• tightly control where your images are being stored
• fully own your images distribution pipeline
• integrate image storage and distribution tightly into your in-house development workflow

Set up a registry proxy cache with Docker open source registry

https://blog.docker.com/2015/10/registry-proxy-cache-docker-open-source/

Configure Docker registry mirror with systemd

Many Linux distributions use systemd to start the Docker daemon. In this post we learn configure Docker registry mirror with systemd.

• Edit docker.service systemd unit:

$ sudo systemctl edit docker.service

This create a file in /etc/systemd/system/docker.service.d/ folder with name like .override.conf and open it with default text editor
Thanks fzerorubigd for introducing systemctl edit

• In the file, add this lines

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H fd:// --registry-mirror=https://example.com:5000

• Flush the change with:

$ sudo systemctl daemon-reload

• Restart Docker with:

$ sudo systemctl restart docker

• Check if the change is successfully applied with $ systemctl status docker, your config should appear somewhere in the command output.

from https://medium.com/@arastu/add-docker-registry-mirror-to-docker-daemon-on-systemd-ubuntu-16-04-d9a224be4d86