Add multiple insecure registries support

[orangedeng]

Add flag daemon.insecure-registry to indecate multiple insecure registry. Append insecure registry parameters when starting daemon.

[tboerger]

There is already an insecure flag.

[eliandoran]

Hello, @tboerger .

I have come across exactly across a scenario where I needed this PR instead of the insecure flag.

Consider the following scenario:

• We have two different Docker repositories, both insecure (run on a local network only): orange.example.com and pear.example.com.
• The docker image that we want to build needs an image from the insecure orange.example.com.
• The docker image that we want to build has to be published to the insecure pear.example.com.

As such, the Dockerfile contains:

FROM orange.example.com/my-secret-base-image

The build step in .drone.yml contains:

- name: deploy-tag
  image: "drone-docker"
    settings:
      repo: "pear.example.com/${DRONE_REPO_NAME}"
      registry: "pear.example.com"
      insecure: true

On a first glance, this should work: we want to publish to pear.example.com which is marked with the insecure flag. Upon publishing you get:

Get "https://orange.example.com/v2/": dial tcp 1.2.3.4:443: connect: connection refused

The reason? The insecure flag only adds the following --insecure-registry flag:

/usr/local/bin/dockerd --data-root /var/lib/docker --host=unix:///var/run/docker.sock --insecure-registry pear.doran.xyz

Basically, the insecure flag would only add the registry we are deploying to, but not any registries our image might depend on.

With this PR, I was able to fix this issue by changing the .drone.yml as follows:

- name: deploy-tag
  image: "drone-docker"
    settings:
      repo: "pear.example.com/${DRONE_REPO_NAME}"
      registry: "pear.example.com"
      insecure_registry: [ "orange.example.com" ]
      insecure: true

This will cause the build to succeed:

/usr/local/bin/dockerd --data-root /var/lib/docker --host=unix:///var/run/docker.sock --insecure-registry pear.doran.xyz --insecure-registry orange.doran.xyz

Personally, I think this is a very useful addition to the plugin as I came across a specific use case for this. However, I would understand that it is a less frequent use case.