Push the Docker image to ghcr.io on all pushes to main (#125)

Author: ldionne

.github/workflows/build-docker.yaml

@@ -30,6 +30,6 @@ jobs:
       id: push
       uses: docker/build-push-action@263435318d21b8e681c14492fe198d362a7d2c83
       with:
-        push: ${{ startsWith(github.ref, 'refs/tags/') }} # only push to ghcr.io on tags
+        push: ${{ github.ref == 'refs/heads/main' }} # only push to ghcr.io on the main branch
         tags: ghcr.io/${{github.repository}}:latest
         context: . # use the current directory as context, as checked out by actions/checkout -- needed for setuptools_scm