Can't deploy to ACA-based Function App

Using the following YAML

```yaml
env:
container_image: 'xxx.azurecr.io/xxx:${{ github.sha }}'

...

- name: 'Sign in via Azure CLI'
uses: azure/login@v2
with:
creds: ${{ secrets.AZURE_CREDENTIALS }}

- uses: azure/docker-login@v1
with:
login-server: xxx.azurecr.io
username: ${{ secrets.REGISTRY_USERNAME }}
password: ${{ secrets.REGISTRY_PASSWORD }}

- name: Build and push Docker image
run: |
pushd xxx.sh
docker build -q -t $container_image .
docker push -q $container_image

- name: Deploy to Azure Web App
uses: azure/webapps-deploy@v3
with:
app-name: 'xxx'
images: $container_image
```

which maps directly to the docs [here](https://github.com/Azure/webapps-deploy#sample-workflow-to-build-and-deploy-a-nodejs-app-to-containerized-webapp-using-azure-service-principal),

I get the following error during deployment: