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

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• 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.
• 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]

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 ⚠️

Additional details and impacted files

@@            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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[achimnol]

Could we add some test cases by providing mocked docker client configurations?