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>

.gitignore

@@ -1,6 +1,6 @@
 # Binaries
-/docker-migrate
-/docker-migrate.exe
+/dockmv
+/dockmv.exe
 /dist/
 *.test
 *.test.exe