Support API version >=1.25 for Docker on Windows support

[schmichael]

Docker on Windows adds some complexity over previous Docker releases as it supports two modes of operation: Linux or Windows containers. Images are OS specific. In order to determine if an image will run, two new pieces of metadata are needed:

• Image.Os (from GET /images/(name)/json has been around since at least API version 1.23 but is missing from the struct
• DockerInfo.Isolation was added to GET /info in API version 1.25 - struct
  • I don't see it in the docs, but hyperv seems to mean Windows mode, and "" (a blank string) means Linux mode. (This is a bit confusing since Microsoft's documentation lists HyperV and Windows Server Containers as 2 different modes which appear to be distinct from Docker's 2 modes.)

As we need these API changes for Nomad (and maybe more I haven't discovered yet) I'll try to submit a PR, but we don't have this particular issue prioritized yet.

[fsouza]

@schmichael I can take a look at this before the weekend, but let me know if you start working on it before :)

[schmichael]

Thanks! I'll make sure to comment here before starting work, but I doubt it'd be this week anyway.

[fsouza]

@schmichael I just pushed the change! :)

[schmichael]

Looks great! Thanks!