fix(ci): Use docker image with pnpm preinstalled

.drone.yml

@@ -8,16 +8,15 @@ platform:
 
 steps:
 - name: Install dependencies
-  image: node:14-alpine
+  image: gplane/pnpm:14-alpine
   commands:
-    - yarn global add pnpm
     - pnpm install --frozen-lockfile
 - name: Check formatting
-  image: node:14-alpine
+  image: gplane/pnpm:14-alpine
   commands:
     - pnpm format:check
 - name: Build site
-  image: node:14-alpine
+  image: gplane/pnpm:14-alpine
   commands:
     - pnpm build
 - name: Deploy site