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>

Makefile

@@ -1,6 +1,6 @@
 VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
 LDFLAGS := -s -w -X main.version=$(VERSION)
-BIN     := docker-migrate
+BIN     := dockmv
 OUT     := dist
 
 # PowerShell 7+. The publish path lives in scripts/*.ps1 because uploading a
@@ -47,7 +47,7 @@ fmt:
 	cd web && npx tsc -b --noEmit
 
 docker:
-	docker build --build-arg VERSION=$(VERSION) -t docker-migrate:$(VERSION) -t docker-migrate:latest .
+	docker build --build-arg VERSION=$(VERSION) -t dockmv:$(VERSION) -t dockmv:latest .
 
 release: ui
 	@mkdir -p $(OUT)