registry auth fallback bugfix (backport #672)

[mergify]

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• Commit(s) and code follow the repositories guidelines.
• Test(s) have been added or updated to support these change(s).
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Associated Links:

Types of Changes:

• Bugfix

Proposed Changes:

Fixes registry credential resolution during hauler store copy registry://, add, and sync when running in environments without a $HOME or $DOCKER_CONFIG set. hauler login has a fallback for this but go-containerregistry doesn't inherently look in the fallback location.

• pkg/content/dockerconfig.go (new): Adds SetDefaultDockerConfig(), which defaults $DOCKER_CONFIG to the same directory hauler login writes to (<home>/.docker) so go-containerregistry's DefaultKeychain can find credentials even when $HOME is unset. Replicates docker/cli's config.Dir()/getHomeDir() resolution — including the /etc/passwd fallback via os/user.Current() — using only the standard library. An explicitly set DOCKER_CONFIG always wins.
• cmd/hauler/cli/cli.go: Invokes SetDefaultDockerConfig() in the root PersistentPreRun, logging the defaulted path at debug level.
• pkg/content/registry.go: Stops silently swallowing credential-resolution failures. Registry host parse errors, keychain resolve errors, and authorization read errors now return wrapped errors instead of falling back to anonymous auth; only a genuine Anonymous result yields empty credentials.

Verification/Testing of Changes:

1. With credentials stored via hauler login <registry> and $HOME unset, run hauler store copy registry://<registry>/... and confirm the push authenticates instead of failing anonymously.
2. Run with --log-level debug and confirm the defaulted $DOCKER_CONFIG to [...] line appears.
3. Confirm an explicitly set $DOCKER_CONFIG is left unchanged.

Additional Context:

Small, targeted bugfix. The previous code returned anonymous credentials on any resolution error, masking real auth failures; this makes credential lookup match hauler login's write location and surfaces genuine errors.

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This is an automatic backport of pull request #672 done by Mergify.

[mergify]

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Changes to be committed:
	new file:   pkg/content/dockerconfig.go
	modified:   pkg/content/registry.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   cmd/hauler/cli/cli.go

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