feat(docker): add repo_region (auto/official/cn) + point service at canonical path

Two changes:

1) Docker generator: introduce 'repo_region' select field with three modes:
     auto     — probe download.docker.com (5s timeout); fall back to Tuna
                mirror if unreachable. Default.
     official — force download.docker.com URLs (GPG + apt/yum repo).
     cn       — force mirrors.tuna.tsinghua.edu.cn/docker-ce URLs.
   The apt/yum repo body now uses ${DOCKER_REPO_APT}/${DOCKER_REPO_YUM}
   variables instead of hardcoded URLs. yum/dnf uses 'command -v
   yum-config-manager' to pick the right tool (dnf-only distros like
   Rocky/Alma now skip the failed yum-config-manager probe).

   Tuna URLs verified reachable: curl -fsSI returned 200 for both
   linux/ubuntu/gpg and linux/centos/gpg. registry_mirror default also
   switched to docker.mirrors.ustc.edu.cn (was tencent cloud).

2) shell-gen.service: point WorkingDirectory and ExecStart at
   /fs/1000/ftp/Project/shell-gen (the canonical git checkout) instead
   of /root/shell-gen. The /root/ copy was a stale duplicate that
   caused previous fixes to silently not reach the running web UI.
   /root/shell-gen is preserved untouched (user has local artifacts
   there: vnc_*.sh, k8s/, deliverables/, etc).

Verified:
  - bash -n OK on all 3 region variants
  - region=auto with reachable overseas: picks official
  - region=auto with overseas-blocked (curl stubbed to fail):
    DOCKER_REPO_APT/YUM correctly fall back to tuna.tsinghua,
    warning emitted on stderr
  - region=official: forces official URLs, no probe
  - region=cn: forces Tuna URLs, no probe
  - 7 other sampled generators unchanged (memcached, wireguard,
    rabbitmq, chrony, mongodb, nginx, redis all bash -n OK,
    $PKG_INSTALL count 0)

shell-gen.service

@@ -5,8 +5,8 @@ After=network.target
 [Service]
 Type=simple
 User=root
-WorkingDirectory=/root/shell-gen
-ExecStart=/usr/bin/python3 /root/shell-gen/app.py
+WorkingDirectory=/fs/1000/ftp/Project/shell-gen
+ExecStart=/usr/bin/python3 /fs/1000/ftp/Project/shell-gen/app.py
 Restart=on-failure
 RestartSec=3
 Environment=PORT=5099