Use Alpine image instead of dotnet for zipping files

.drone.yml

@@ -5,8 +5,10 @@ type: docker
 
 steps:
 - name: build
-  image: mcr.microsoft.com/dotnet/sdk:5.0
+  image: alpine
   commands:
+  - curl -LJO https://dot.net/v1/dotnet-install.sh
+  - ./dotnet-install.sh -c 5.0
   - ./build.sh
   - ./zip-files.sh