[GithubActions] Fixing workflow syntax error (see actions/runner#2372)

Author: Murilo Marinho

.github/workflows/docker_build.yml

@@ -1,12 +1,9 @@
 name: Build Docker Image for CoppeliaSim EDU
 on: [push, workflow_dispatch]
 
-env:
- BRANCH_NAME: ${{ github.head_ref || github.ref_name }}
-
 jobs:
   build_image:
-    name: Build Docker Image for ${{env.BRANCH_NAME}}
+    name: Build Docker Image for ${{ github.head_ref || github.ref_name }}
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
@@ -22,11 +19,11 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
-      - name: Build and push CoppeliaSim for ${{env.BRANCH_NAME}}
+      - name: Build and push CoppeliaSim for ${{ github.head_ref || github.ref_name }}
         uses: docker/build-push-action@v6
         with:
           platforms: linux/amd64
           push: true
           file: coppeliasim/Dockerfile
           tags: | 
-              ${{ vars.DOCKER_USERNAME }}/coppeliasim:${{env.BRANCH_NAME}}
+              ${{ vars.DOCKER_USERNAME }}/coppeliasim:${{ github.head_ref || github.ref_name }}