fix: explain uninitialized pass credential store

[bghira]

Closes #842

Summary

• capture stderr from Docker credential helpers when storing login credentials
• recognize the pass not initialized failure and explain how to create a GPG key and initialize pass
• preserve other credential-helper stderr in the returned error
• add a regression test using a fake credential helper

Validation

• go test ./pkg/...
• go vet ./pkg/docker
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.10.1 run ./pkg/docker/...
• git diff --check

[anish-sahoo]

lgtm, ci breaks