registry auth fallback bugfix

[amartin120]

Please check below, if the PR fulfills these requirements:

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

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.

[atoy3731]

LGTM.

[amartin120]

@Mergifyio backport release/2.0

[mergify]

backport release/2.0

✅ Backports have been created

Details

• #684 registry auth fallback bugfix (backport #672) has been created for branch release/2.0 but encountered conflicts

Cherry-pick of 6f5d767 has failed:

On branch mergify/bp/release/2.0/pr-672
Your branch is up to date with 'origin/release/2.0'.

You are currently cherry-picking commit 6f5d767.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

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

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally