Make datalad_container.adapters.docker save assume latest if no image version given

[jwodder]

Closes #105.

[codeclimate]

Code Climate has analyzed commit 618b9af and detected 7 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Security	7

View more on Code Climate.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07 🎉

Comparison is base (c3bb747) 93.82% compared to head (618b9af) 93.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   93.82%   93.90%   +0.07%     
==========================================
  Files          19       19              
  Lines         956      968      +12     
==========================================
+ Hits          897      909      +12     
  Misses         59       59              

Impacted Files	Coverage Δ
datalad_container/adapters/docker.py	86.60% <100.00%> (+0.24%)	⬆️
datalad_container/adapters/tests/test_docker.py	95.18% <100.00%> (+0.66%)	⬆️

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[yarikoptic]

Looks great. If tests pass -- can be merged

[github-actions]

PR released in 1.2.0