Cache base image metadata in-memory

[jonjohnsonjr]

This saves a roundtrip to the registry if we ever use the same base
image for multiple builds in a single invocation.

This also sets us up for using an on-disk cache for image metadata to
speed things up even further.

[codecov-commenter]

Codecov Report

Merging #525 (4339c5f) into main (6ff3469) will increase coverage by 0.04%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #525      +/-   ##
==========================================
+ Coverage   50.58%   50.63%   +0.04%     
==========================================
  Files          40       40              
  Lines        2040     2044       +4     
==========================================
+ Hits         1032     1035       +3     
+ Misses        834      833       -1     
- Partials      174      176       +2     

Impacted Files	Coverage Δ
pkg/commands/config.go	57.37% <50.00%> (+1.23%)	⬆️

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