Add support for Docker context endpoints

[jrfk]

What do these changes do?

Add support for Docker context endpoints.

• Read files such as ~/.docker/context/meta.json and set the socket
  • DOCKER_CONTEXT will be prioritized if it is set.

Are there changes in behavior for the user?

No impacts to existing users.

Related issue number

resolves #811

Checklist

• [x] I think the code is well written
• [ ] Unit tests for the changes exist
• [ ] Documentation reflects the changes
• [x] If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• [x] Add a new news fragment into the changes folder
  • name it <issue_id>.<type> for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
  • .feature: Signifying a new feature.
  • .bugfix: Signifying a bug fix.
  • .doc: Signifying a documentation improvement.
  • .removal: Signifying a deprecation or removal of public API.
  • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.82353% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.06%. Comparing base (1a33187) to head (98e6438). Report is 1 commits behind head on master.

Files	Patch %	Lines
aiodocker/docker.py	58.82%	5 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #854 +/- ## ========================================== - Coverage 81.34% 81.06% -0.28% ========================================== Files 24 24 Lines 1372 1389 +17 Branches 245 250 +5 ========================================== + Hits 1116 1126 +10 - Misses 170 175 +5 - Partials 86 88 +2 ```

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