Fix repo2docker version (#139)

It seems that a recent update to Mamba is causing a problem when building the
Docker image from the repo via repo2docker. See jupyterhub/mybinder.org-deploy#2714.
This PR fixes the version of `repo2docker` to the last one that didn't have this problem,
and the image is built correctly.

.github/workflows/build-docker-image.yml

@@ -27,3 +27,4 @@ jobs:
                   DOCKER_PASSWORD: ${{ secrets.GITHUB_TOKEN }}
                   DOCKER_REGISTRY: ghcr.io
                   IMAGE_NAME: ${{ github.repository }}
+                  FORCE_REPO2DOCKER_VERSION: jupyter-repo2docker==2023.06.0