Update azure-pipelines.yml to build the docker image.

Author: anandakumarpalanisamy

azure-pipelines.yml

@@ -9,11 +9,13 @@ trigger:
 pool:
   vmImage: ubuntu-latest
 
-steps:
-- script: echo Hello, world!
-  displayName: 'Run a one-line script'
+variables:
+  imageName: 'pipelines-javascript-docker'
 
-- script: |
-    echo Add other tasks to build, test, and deploy your project.
-    echo See https://aka.ms/yaml
-  displayName: 'Run a multi-line script'
+steps:
+- task: Docker@2
+  displayName: Build an image
+  inputs:
+    repository: $(image)
+    command: build
+    Dockerfile: app/Dockerfile