update image-publishing workflow after default branch rename

Author: minrk

.github/workflows/image.yaml

@@ -19,7 +19,7 @@ jobs:
           image="jupyter/nbviewer:$short_sha"
           echo "::set-output name=image::$image"
           tags="$image"
-          if "${{ github.ref_name }}" == "master" ]]; then
+          if "${{ github.ref_name }}" == "main" ]]; then
             tags="jupyter/nbviewer:latest $tags"
           fi
           echo "::set-output name=tags::$tags"
@@ -28,7 +28,7 @@ jobs:
         run: echo ${{ steps.vars.outputs.sha_short }}
 
       - name: Login to docker hub
-        if: github.ref_name == 'master'
+        if: github.ref_name == 'main'
         uses: docker/login-action@v1
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
@@ -49,10 +49,10 @@ jobs:
           docker run --rm -i ${{ steps.vars.outputs.image }} python3 -m nbviewer --help-all
 
       - name: Push
-        if: github.ref_name == 'master'
+        if: github.ref_name == 'main'
         uses: docker/build-push-action@v2
         with:
           context: .
           platforms: linux/amd64
-          push: ${{ github.ref_name == 'master' }}
+          push: ${{ github.ref_name == 'main' }}
           tags: ${{ steps.vars.outputs.tags }}