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.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/arescom/docker-migrate/internal/spec"
+	"github.com/arescom/dockmv/internal/spec"
 )
 
 // renderInstaller generates the shell script shipped inside a migration
@@ -451,14 +451,14 @@ seed_readonly() { # container image destination relpath compressed
   store="$(resolve_mount "$1" "$3")" || { err "cannot resolve storage behind $3"; return 1; }
   base="${3##*/}"
   stage="dm-stage-$$-${RANDOM}"
-  mountat="/__docker_migrate/$base"
+  mountat="/__dockmv/$base"
   $DOCKER create --name "$stage" --volume "$store:$mountat" "$2" >/dev/null \
     || { err "could not create staging container for $3"; return 1; }
   local rc=0
   if [ "$5" = 1 ]; then
-    gzip -dc -- "$PKGDIR/$4" | $DOCKER cp -a - "$stage:/__docker_migrate" || rc=$?
+    gzip -dc -- "$PKGDIR/$4" | $DOCKER cp -a - "$stage:/__dockmv" || rc=$?
   else
-    $DOCKER cp -a - "$stage:/__docker_migrate" < "$PKGDIR/$4" || rc=$?
+    $DOCKER cp -a - "$stage:/__dockmv" < "$PKGDIR/$4" || rc=$?
   fi
   $DOCKER rm -f "$stage" >/dev/null 2>&1 || true
   if [ "$rc" != 0 ]; then