fix: fall back after image transport init failure

[Eljees]

Summary

• continue through configured image transports when a source opens but cannot be initialized
• close the failed source before attempting the next transport
• add a regression test with a failing first registry and a valid fallback registry

Root cause

Dockle tries docker-daemon: before docker://. For a locally pulled image referenced by its registry manifest digest, the daemon source opens but image.FromSource rejects the locally reconstructed manifest. newSource returned that error immediately instead of trying the registry transport already present in the fallback list.

Testing

• CGO_ENABLED=0 go test ./pkg/deckoder/extractor/... -count=1
• CGO_ENABLED=0 go vet ./pkg/deckoder/extractor/...
• reproduced with local alpine@sha256:5b10f432ef3da1b8d4c7eb6c487f2f5a8f096bc91145e68878dd4a5019afde11: current master fails with Manifest does not match provided manifest digest; this branch completes the scan

Fixes #262