Fix base image: use mirror direct path, switch to node:20-alpine

Mirror exposes images as mirror.soroushasadi.com/<image> directly,
not via /repository/docker-group/ path. Also node:20-alpine is already
cached on the server; node:20-slim was never pulled.

Dockerfile: apk instead of apt-get, addgroup/adduser instead of
groupadd/useradd (Alpine BusyBox compatibility).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -6,7 +6,7 @@ services:
       context: .
       dockerfile: Dockerfile
       args:
-        NODE_IMAGE: ${NODE_IMAGE:-mirror.soroushasadi.com/repository/docker-group/node:20-slim}
+        NODE_IMAGE: ${NODE_IMAGE:-mirror.soroushasadi.com/node:20-alpine}
         NPM_TOKEN: ${NPM_TOKEN:-}
     image: soroushasadi-site:latest
     container_name: soroushasadi-site