fix: honor standard Docker host environment

[Eljees]

Summary

• accept the standard DOCKER_HOST environment variable as a source for Dockle's daemon host option
• preserve the existing precedence of explicit --host and DOCKLE_HOST
• add table-driven coverage for CLI, both environment variables, XDG, and default socket selection

Root cause

The documented GitLab Docker-in-Docker setup exports DOCKER_HOST=tcp://docker:2375 for Docker clients. Dockle only read DOCKLE_HOST and otherwise forced unix:///var/run/docker.sock, so it could not see the locally built image and fell back to an unauthorized Docker Hub lookup.

Testing

• CGO_ENABLED=0 go test ./pkg -count=1
• CGO_ENABLED=0 go vet ./pkg

Fixes #263