[AIRFLOW-3193] Pin docker requirement version

[deagon]

Since the change of AIRFLOW-3203, the dockerpy client has upgraded
actually. Therefore, the requirements config should upgrade also to
avoid potentially misleading.

@XD-DENG @Fokko

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[deagon]

AIRFLOW-3203 linked to #4049.

[XD-DENG]

Thanks @deagon . You're correct: one part of my PR (#4049) was to address the breaking change in docker==3.0.0 and we do need to pin the version for it.

This PR looks sensible to me.

[codecov-io]

Codecov Report

Merging #4130 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4130      +/-   ##
==========================================
+ Coverage   76.66%   76.67%   +<.01%     
==========================================
  Files         199      199              
  Lines       16212    16212              
==========================================
+ Hits        12429    12430       +1     
+ Misses       3783     3782       -1

Impacted Files	Coverage Δ
airflow/models.py	92.09% <0%> (+0.04%)	⬆️

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[XD-DENG]

Hi @ashb , would you mind to add this commit into 1.10.1?

Understand that my PR #4049 ([AIRFLOW-3203] Fix DockerOperator & some operator test) is already cherry-picked into branch v1-10-test. The change made in that PR is partially meant to address a breaking change in Python package docker==3.0.0. But I missed to pin the docker version in that commit. This may result in issues for users who have docker < 3.0.0 when they upgrade to 1.10.1.

Cheers.

[ashb]

@XD-DENG Ah cool. will do!