Attempt 2: Check for Podman's auth in DefaultKeychain

[imjasonh]

This is a second attempt after #1181, reverted in #1184 due to #1183

The bug seems to be that if $DOCKER_CONFIG was unset, we would never look for $HOME/.docker/config.json, which config.Load does.

The order should be:

1. look for $HOME/.docker/config.json <-- missing before; very common
2. look for $DOCKER_CONFIG/config.json
3. look for $XDG_RUNTIME_DIR/containers/auth.json (Podman auth)

This is now reflected in this PR, with a unit test that builds up each option from 3->1 and checks that the correct auth config is used.

I'm open to more ideas about how to test this better, in case there are more edge cases I'm still not considering.

[codecov-commenter]

Codecov Report

Merging #1185 (5f6e919) into main (bce5496) will decrease coverage by 0.02%.
The diff coverage is 74.28%.

@@            Coverage Diff             @@
##             main    #1185      +/-   ##
==========================================
- Coverage   75.24%   75.21%   -0.03%     
==========================================
  Files         108      108              
  Lines        7856     7887      +31     
==========================================
+ Hits         5911     5932      +21     
- Misses       1379     1385       +6     
- Partials      566      570       +4     

Impacted Files	Coverage Δ
pkg/authn/keychain.go	78.94% <74.28%> (-21.06%)	⬇️
pkg/v1/remote/multi_write.go	63.90% <0.00%> (+0.97%)	⬆️

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