Fix Linux CI to actually run inside a docker container (dotnet#574)

Author: safern

.vsts-dotnet-ci.yml

@@ -1,15 +1,9 @@
-resources:
-  containers:
-  - container: centos7
-    image: microsoft/dotnet-buildtools-prereqs:centos-7-b46d863-20180719033416
-
 phases:
 - template: /build/ci/phase-template.yml
   parameters:
     name: Linux
     buildScript: ./build.sh
-    queue:
-      container: centos7
+    dockerImage: microsoft/dotnet-buildtools-prereqs:centos-7-b46d863-20180719033416
 
 - template: /build/ci/phase-template.yml
   parameters:

build/ci/phase-template.yml

@@ -1,13 +1,17 @@
 parameters:
   name: ''
   buildScript: ''
+  dockerImage: ''
   queue: {}
 
 phases:
   - phase: ${{ parameters.name }}
     variables:
       _buildScript: ${{ parameters.buildScript }}
       _phaseName: ${{ parameters.name }}
+      # if dockerImage is not equal to '' then run under docker container
+      ${{ if ne(parameters.dockerImage, '') }}:
+        _PREVIEW_VSTS_DOCKER_IMAGE: ${{ parameters.dockerImage }}
     queue:
       parallel: 2
       matrix: