try using bash to check default branch

.github/workflows/docker.yml

@@ -27,6 +27,14 @@ jobs:
     steps:
       - name: Check out code
         uses: actions/checkout@v4
+
+      - name: Set default branch variable
+        run : |
+          if [[ $GITHUB_REF_NAME == 'main' ]]; then
+              echo "IS_MAIN_BRANCH=true" >> "$GITHUB_ENV"
+          else
+              echo "IS_MAIN_BRANCH=false" >> "$GITHUB_ENV"
+          fi
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -61,7 +69,7 @@ jobs:
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha,scope=ubicloud-${{ matrix.runner }}
           cache-to: type=gha,mode=max,scope=ubicloud-${{ matrix.runner }}
-          outputs: type=image,name=${{ env.IMAGE_NAME }},push-by-digest={{is_default_branch}},name-canonical={{is_default_branch}},push={{is_default_branch}}
+          outputs: type=image,name=${{ env.IMAGE_NAME }},push-by-digest=${{env.IS_MAIN_BRANCH}},name-canonical=${{env.IS_MAIN_BRANCH}},push=${{env.IS_MAIN_BRANCH}}
           provenance: false
 
       - name: Export digest