Add docker input server version

[danielnelson]

closes #4029

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

@rdxmb Can you review?

[rdxmb]

Oh, I am not a Go-developer, so I cannot make a "real review". But, I can have a look at it, of course.

What I think about is if we need the tag really with all measurements. Having it with docker could be enough, I guess.

[danielnelson]

Hmm, maybe it should be a string field then, what sort of queries are you planning to use?

[rdxmb]

As mostly the strings I use are stored as tags, I did not have the idea to use a string field for that.

However, for anybody else a use-case could also be to get what engine-version is running under a container. For that, your PR would be the best.

My personal use case is to just check if the docker-engines are up-to-date. So for now I am very flexible about the query.

[danielnelson]

I think having it as a tag to all of the docker_* measurements will end up be the least confusing.