Move credentials after build dependencies, controlled by calibanconfig

[sritchie]

Dependencies are much more expensive to change than credentials. This PR moves the credential injection after dependency fetching.

This is a decent idea, I think, BUT it has a problem. If you have private dependencies in a google Source Cloud Repository, you need the credentials to come before the pip install so that git can use gcloud for authentication.

@ajslone , I'm going to vote that we:

• make this change here, but only after
• we add a setting in .calibanconfig.json that allows you to move this earlier if you need it there.

I am a little worried at configuration options starting to increase. What do you think?

[codecov[bot]]

Codecov Report

Merging #10 into master will decrease coverage by 0.09%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   43.73%   43.64%   -0.10%     
==========================================
  Files          20       20              
  Lines        3114     3114              
==========================================
- Hits         1362     1359       -3     
- Misses       1752     1755       +3     

Impacted Files	Coverage Δ
caliban/docker.py	25.32% <0.00%> (ø)
caliban/gke/utils.py	73.38% <0.00%> (-1.15%)	:arrow_down:

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