Rename product to dockmv (container, image, module, env vars)

Container/image/service name, Go module path, CLI binary name, and
DOCKER_MIGRATE_* env vars still used the old working name; the project
is branded DockMV everywhere else (README, logo, Gitea repo).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/migrate/installer_test.go

@@ -11,7 +11,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/arescom/docker-migrate/internal/spec"
+	"github.com/arescom/dockmv/internal/spec"
 )
 
 func buildPrepared(t *testing.T, c *spec.Container, vols []spec.Volume, nets []spec.Network) *Prepared {