Defer to builder run image mirror if not publishing (daemon case)

[dfreilich]

Signed-off-by: David Freilich [email protected]

Summary

This work attempts to make the selection of run-image-mirrors more intuitive. Previously, our selection of a run image mirror looked like this:

1. run image provided with pack build --run-image
2. run image mirror in the same registry as the app you are trying to build (so, for instance, pack build test-app will look for a run image mirror in dockerhub, while pack build gcr.io/test-app will look for a run image mirror in gcr.io)
3. run image mirrors configured using pack set-run-image-mirror
4. run image mirrors defined on the builder/stack

In practice, this was confusing to users (see #712, and a conversation on slack here). While the above priorities made sense when thinking about publishing images (if you are publishing, we want to minimize the number of bits you have to stream, so using a run image mirror in that registry would be optimal), they were suboptimal when users were doing local development, where users would have expected the run image to be in the same registry as the builder.

As such, this PR addresses changes priority (2) above, adjusting between selecting a run image mirror:

• in the same registry as the app, if you are publishing (running pack build --publish, referred to as a non-daemon case)
• in the same registry as the builder, if you are doing local development (running pack build, referred to as the daemon case)

Output

Before

$ pack build

$ pack build --publish

After

$ pack build

$ pack build --publish

Documentation

• Should this change be documented?
  • Yes, in the release notes

Related

Resolves #712

[codecov]

Codecov Report

Merging #783 into main will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #783      +/-   ##
==========================================
+ Coverage   74.76%   74.85%   +0.09%     
==========================================
  Files          77       77              
  Lines        5174     5180       +6     
==========================================
+ Hits         3868     3877       +9     
+ Misses       1001      999       -2     
+ Partials      305      304       -1     

Flag	Coverage Δ
#os_linux	77.33% <100.00%> (+0.08%)	⬆️
#os_macos	73.25% <100.00%> (+0.08%)	⬆️
#os_windows	73.13% <100.00%> (+0.09%)	⬆️
#unit	74.85% <100.00%> (+0.09%)	⬆️

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