Docker build updates - zsh fixes

[wtgee]

• Fix zsh install path. The oh-my-zsh repo has moved into a new top-level project so the url has changed. Make it an ARG while we're at it.
• Fix test output return, closes #247
• Default to develop tag for cloudbuild images.

[codecov[bot]]

Codecov Report

Merging #256 into develop will decrease coverage by 0.41%. The diff coverage is 58.06%.

@@             Coverage Diff             @@
##           develop     #256      +/-   ##
===========================================
- Coverage    79.02%   78.60%   -0.42%     
===========================================
  Files           49       49              
  Lines         3089     3048      -41     
  Branches       273      273              
===========================================
- Hits          2441     2396      -45     
- Misses         607      611       +4     
  Partials        41       41              

Impacted Files	Coverage Δ
src/panoptes/utils/config/cli.py	46.15% <0.00%> (-0.69%)	:arrow_down:
src/panoptes/utils/config/server.py	18.89% <0.00%> (-2.48%)	:arrow_down:
src/panoptes/utils/images/__init__.py	64.63% <0.00%> (ø)
src/panoptes/utils/config/client.py	90.19% <100.00%> (ø)
src/panoptes/utils/config/helpers.py	100.00% <100.00%> (ø)
src/panoptes/utils/utils.py	92.78% <100.00%> (+0.15%)	:arrow_up:
tests/config/test_config_cli.py	28.12% <0.00%> (-7.99%)	:arrow_down:
conftest.py	90.36% <0.00%> (-1.40%)	:arrow_down:
...utils/serial_handlers/protocol_arduinosimulator.py	17.52% <0.00%> (-1.28%)	:arrow_down:
... and 13 more

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