But I think it's desirable that docker configuration is set in the instance of struct Docker (or in docker module layer).

I do see how reading DOCKER_API_VERSION environment variable in a module responsible for HTTP communication is somewhat ugly, but at the same time I'm not sure how I'd go about this one without introducing unjustified code duplication/complication.

And for avoiding break existing code, it is preferable to allow users use dockworker without api version. How do you think?

Here are some facts:

Docker started supporting versioned endpoints even before 1.0 release, in version 0.3.3, which was like 6 years ago, see moby/moby@faae7220c019882a2160aeeexplicitlya6bebc46c15d702be.

In the very same commit there's a check introduced to make sure Docker daemon won't talk to a client with a higher version:

https://github.com/moby/moby/blob/faae7220c019882a2160aeea6bebc46c15d702be/api.go#L653-L655

This check indeed won't trigger when we don't specify a version, but allowing users to disable versioned endpoints and hoping for the best sounds like a bad idea.

Also, Docker API version 1.25 requires that you use versioned endpoints, but AFAIU this won't be in effect until this minimum version is explicitly enforced by the daemon:

https://github.com/moby/moby/blob/8e610b2b55bfd1bfa9436ab110d311f5e8a74dcb/api/common_unix.go#L6

Perhaps this PR needs a little more thought from the GH-21 point of view, i.e. how exactly version negotiation and library method versioning are going to be implemented.