Merge infrastructure changes from nightly (dotnet#823)

Author: MichaelSimons

.gitignore

@@ -24,3 +24,6 @@ artifacts/
 # User-specific files
 *.suo
 *.user
+
+# ImageBuilder directory
+.Microsoft.DotNet.ImageBuilder

.vsts-pipelines/builds/microsoft-dotnet-nightly.yml

@@ -2,9 +2,9 @@ trigger:
   batch: true
   branches:
     include:
-      - nightly
+    - nightly
 variables:
   manifest: manifest.json
   repo: dotnet-nightly
-phases:
-  - template: ../phases/build-all.yml
+jobs:
+- template: ../jobs/build-test-publish-repo.yml