fix(ci): pnpm via npm install (skip corepack) — Dockerfile + build-docs job

Corepack может hang в CI runner на integrity-check либо non-TTY prompt
при первом activate. Заменяем на прямой npm install с pinned version
9.15.4 (latest stable v9, lockfile compat: lockfileVersion '9.0').

Изменения:
- ordinis-admin-ui/Dockerfile: corepack → npm install -g pnpm@9.15.4
- docs/Dockerfile: same swap
- .gitlab-ci.yml build-docs: same swap

Verified locally: docker build ordinis-admin-ui за 23 sec, no hang.

.gitlab-ci.yml

@@ -126,8 +126,8 @@ build-docs:
   stage: build
   image: repo.nstart.cloud/library/node:20-alpine
   before_script:
-    - corepack enable
-    - corepack prepare pnpm@9.0.0 --activate
+    # npm install pnpm — corepack может hang на CI runner integrity checks.
+    - npm install -g --silent pnpm@9.15.4
   script:
     - cd docs
     - pnpm install --frozen-lockfile