add support for DOCKER_API_VERSION

[aripringle]

Fixes #13. The latest version of alpine now has docker 1.11, which supports the DOCKER_API_VERSION environment variable. This change makes that value default to 1.20 (for backwards compatibility), and allows it to be overridden to other versions.

I've tested this on a couple of different docker versions. The default API version (1.20) seems to work fine on a docker 1.11.1 host system, and overriding it to the latest API version (1.23) also works.

Unfortunately, it doesn't seem to quite work for docker 1.7.1 on the host system (API version 1.19). I don't have access to other systems to test, but would be curious if it works on docker 1.8 systems.

[mikljohansson]

@aripringle was there a specific reason to select API version 1.20 as the default instead of 1.21 (Docker 1.9.0+) which supports the docker volume command? We'd like to change the default API version to 1.21 if possible

Edit: actually nevermind, it seems that the docker volume command works regardless of what you set DOCKER_API_VERSION to as long as it's >= 1.12

Best,
Mikael

[aripringle]

Hi @mikljohansson, thanks for the merge. The only reason I set the default API version was to attempt backwards compatibility with the previous build of this project. Otherwise I think it's fine to default to the latest version, or omit the ENV directive altogether so docker uses its default (but could still be overridden of course)