Prune the docker system before running

.github/workflows/dockerimage.yml

@@ -1,27 +1,30 @@
 name: Docker Image CI
 
 on: [push, pull_request]
-      
+
 jobs:
 
   build:
- 
+
     runs-on: ubuntu-latest
- 
+
     steps:
     - uses: actions/checkout@v1
-      
+
     - name: Build the extender-client.jar
       run: (cd client && ../gradlew build)
-      
+
     - name: Test the extender-client.jar
       run: (cd client && ../gradlew test)
-
+
+    - name: Prune Docker system
+      run: docker system prune -f
+
     - name: Build the Docker image
       run: ./server/scripts/build.sh -xtest
       env:
             S3_URL:  ${{ secrets.S3_URL }}
-    
+
     - name: Test the Docker image
       run: ./gradlew test
       env: