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/sshx/docker.go

@@ -7,7 +7,7 @@ import (
 	"io"
 	"strings"
 
-	"github.com/arescom/docker-migrate/internal/spec"
+	"github.com/arescom/dockmv/internal/spec"
 )
 
 // RemoteDocker drives the docker CLI on a target host over SSH. It assumes