Made the async docker client instance private

[stefanoborini]

There is no reason why the ContainerManager should export the docker client publicly.

[codecov-io]

Codecov Report

Merging #360 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #360   +/-   ##
=======================================
  Coverage   95.25%   95.25%           
=======================================
  Files          92       92           
  Lines        3895     3895           
  Branches      253      253           
=======================================
  Hits         3710     3710           
  Misses        127      127           
  Partials       58       58

Impacted Files	Coverage Δ
remoteappmanager/tests/mocking/dummy.py	91.46% <100%> (ø)	✅
...eappmanager/docker/tests/test_container_manager.py	100% <100%> (ø)	✅
remoteappmanager/docker/container_manager.py	85.64% <100%> (ø)	✅

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