update docs to reflect current base images, document how to specify custom base image

[ajslone]

This PR updates our docs to document our current base images, and document how to specify a custom base image in .calibanconfig.json

[codecov]

Codecov Report

Merging #92 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   55.56%   55.53%   -0.04%     
==========================================
  Files          31       31              
  Lines        3180     3180              
==========================================
- Hits         1767     1766       -1     
- Misses       1413     1414       +1     

Impacted Files	Coverage Δ
caliban/platform/gke/util.py	98.13% <0.00%> (-1.12%)	⬇️
caliban/platform/cloud/types.py	89.87% <0.00%> (-0.64%)	⬇️
caliban/util/fs.py	90.56% <0.00%> (+0.94%)	⬆️
caliban/util/auth.py	76.19% <0.00%> (+9.52%)	⬆️

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[sritchie]

Excellent, so good!