fix(ci): install yarn via npm before frontend build (catthehacker image lacks yarn)

.github/workflows/docker-publish.yml

@@ -57,6 +57,7 @@ jobs:
           echo "RAM: ${total_ram_mb}MB — setting Node.js max to ${node_mem}MB"
           export NODE_OPTIONS="--max-old-space-size=${node_mem}"
           cd frontend
+          npm install -g yarn
           yarn install --frozen-lockfile
           yarn locale-compile
           yarn build