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>

.dockerignore

@@ -1,8 +1,8 @@
 .git
 .gitignore
 dist/
-docker-migrate
-docker-migrate.exe
+dockmv
+dockmv.exe
 web/node_modules
 internal/webui/dist
 *.test