Delay updating binder until MCR image is available. (microsoft#287)

* Delay updating binder until MCR image is available.

* Print out diagnostics on spinlock.

* Remove trigger used in tests.

.github/workflows/main.yml

@@ -23,7 +23,7 @@ jobs:
         $Now = [DateTime]::Now;
         $ImageTag = "${{ github.sha }}"
         $RepoName = "public/quantum/samples"
-        
+
         $LocalTag = "${RepoName}:${ImageTag}"
         $RemoteRepo = "${{ secrets.ACR_REGISTRY }}/${RepoName}"
 
@@ -33,14 +33,38 @@ jobs:
         docker tag $LocalTag "${RemoteRepo}:latest"
         docker push "${RemoteRepo}:latest"
       shell: pwsh
+    - name: Wait for image to publish
+      run: |
+        function Get-Tags() {
+          Invoke-WebRequest https://mcr.microsoft.com/v2/quantum/samples/tags/list `
+            | Select-Object -ExpandProperty Content `
+            | ConvertFrom-Json `
+            | Select-Object -ExpandProperty tags `
+            | Write-Output;
+        }
+
+        $ImageAvailable = $false;
+        $ImageTag = "${{ github.sha }}";
+        $CheckInterval = 30; # [seconds]
+        while (-not $ImageAvailable) {
+          if ($ImageTag -in (Get-Tags)) {
+            Write-Host "##[info] Image $ImageTag now available on mcr.microsoft.com, proceeding."
+            $ImageAvailable = $true;
+          } else {
+            Write-Host "##[info] Image $ImageTag not yet available on mcr.microsoft.com, waiting $CheckInterval seconds.";
+          }
+
+          Start-Sleep -Seconds $CheckInterval;
+        }
+      shell: pwsh
     - name: Update Binder configuration
       run: |
         git config --local user.email "[email protected]"
         git config --local user.name "GitHub Action"
         $ThisCommit = "${{ github.sha }}"
         git checkout ⭐binder
         git reset --hard $ThisCommit
-        
+
         $Dockerfile = Get-Content ./Dockerfile;
         $Dockerfile `
           | ForEach-Object {
@@ -55,7 +79,7 @@ jobs:
         git add Dockerfile
         git commit -m "Updated Binder to use $ThisCommit."
       shell: pwsh
-    
+
     - name: Push changes
       uses: ad-m/github-push-action@master
       with: