build: test 1.12.2 with java8

.github/workflows/main.yml

@@ -54,7 +54,7 @@ jobs:
 
     - name: Build for test
       uses: docker/build-push-action@v2.7.0
-      if: github.ref_name == 'master'
+      if: github.ref_name == 'java8'
       with:
         context: .
         platforms: linux/amd64
@@ -70,6 +70,8 @@ jobs:
       if: github.ref_name == 'master'
       run: |
         tests/test.sh
+      env:
+        MINECRAFT_VERSION: 1.12.2
 
     - name: Build and push
       id: docker_build