docker: fix daemon-json template

[etungsten]

Issue number:
N/A

Description of changes:

Author: Erikson Tung <etung@amazon.com>
Date:   Thu Nov 11 15:52:17 2021 -0800

    docker: fix daemon-json template
    
    This fixes rendering of docker/daemon.json when
    container-registry.mirrors is not specified.

Testing done:
Launch instance with the following userdata, no container-registry.mirrors setting:

[settings.host-containers.admin]
enabled = true

[settings.host-containers.control]
enabled=true

Docker daemon.json renders correctly

bash-5.0# cat /etc/docker/daemon.json 
{
  "log-driver": "journald",
  "live-restore": true,
  "max-concurrent-downloads": 10,
  "storage-driver": "overlay2",
  "exec-opts": ["native.cgroupdriver=cgroupfs"],
  "data-root": "/var/lib/docker",
  "selinux-enabled": true,
  "default-ulimits": { "nofile": { "Name": "nofile", "Soft": 1024, "Hard": 4096 } }
}

Launch instance with the following userdata, with container-registry.mirrors setting:

[settings.host-containers.admin]
enabled = true

[settings.host-containers.control]
enabled=true

[[settings.container-registry.mirrors]]
registry = "docker.io"
endpoint = ["https://registry-0.acme.com","https://registry-1.acme.com"]

[[settings.container-registry.mirrors]]
registry = "example"
endpoint = [ "hello", "hellohello"]

Docker daemon.json renders correctly:

bash-5.0# cat /etc/docker/daemon.json 
{
  "log-driver": "journald",
  "live-restore": true,
  "max-concurrent-downloads": 10,
  "storage-driver": "overlay2",
  "exec-opts": ["native.cgroupdriver=cgroupfs"],
  "data-root": "/var/lib/docker",
  "selinux-enabled": true,
  "default-ulimits": { "nofile": { "Name": "nofile", "Soft": 1024, "Hard": 4096 } }
  ,
  "registry-mirrors": ["https://registry-0.acme.com", "https://registry-1.acme.com"]
  }
bash-5.0# 

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[samuelkarp]

LGTM assuming your tests are successful

[zmrow]

LGTM
🥧