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/prepare.go

@@ -8,7 +8,7 @@ import (
 	"path"
 	"strings"
 
-	"github.com/arescom/docker-migrate/internal/spec"
+	"github.com/arescom/dockmv/internal/spec"
 )
 
 // Prepared is one container resolved against the user's selection: the spec as
@@ -189,7 +189,7 @@ func Prepare(
 
 // StagingMountPath is where a read-only destination is mounted inside the
 // temporary staging container used to seed it.
-const stagingRoot = "/__docker_migrate"
+const stagingRoot = "/__dockmv"
 
 // StagingPaths returns the mount point and the extraction directory used when
 // seeding a read-only mount through a staging container. The volume is mounted