2.0.2: Add multi-instance Docker support with SC_DOCKER_ env prefix

Enable running N independent S-Cart/GP247 projects on one host by scoping
the compose project name, every container_name, the image tag and the named
volumes via SC_DOCKER_INSTANCE. Unset it and all identifiers stay byte-for-byte
as before (project "scart"/"scart-prod", "scart-app", "scart-vendor"...), so
existing single-project deployments are unaffected.

Prefix all Docker/host-side .env variables with SC_DOCKER_ so they are easy to
tell apart from the Laravel app config (APP_*, DB_*, MAIL_*...). Renamed:
PHP_VERSION, INSTALL_XDEBUG, XDEBUG_MODE, INSTANCE, APP_PORT, VITE_PORT,
DB_ROOT_PASSWORD, WWWUSER, WWWGROUP. These are consumed only by
docker-compose*.yml and the Dockerfile build args, never by the app itself.

COMPOSE_PROFILES keeps its exact name (Docker Compose reads it directly), and
Laravel-owned DB_HOST/DB_PORT/DB_USERNAME/DB_PASSWORD stay unprefixed.

Docs (DOCKER.md, DOCKER_vi.md, README.md, README_vi.md) updated with a
"run several projects on one host" section and the renamed variables.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>