Adds Microsoft.Docker.Sdk to CLI

This change is to add Sdk.props and Sdk.targets of Microsoft.Docker.Sdk
into CLI. This unblocks the scenario where a VS solution contains a few
.NET Core projects as well as a docker-compose.dcproj project and people
want to build the solution from command line with .NET Core CLI. With
the Sdk.props and Sdk.targets being present in CLI, building
docker-compose.dcproj becomes no-op so it won't block building the other
.NET Core projects.

TestAssets/TestProjects/docker-compose/docker-compose.dcproj

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project Sdk="Microsoft.Docker.Sdk">
+  <PropertyGroup Label="Globals">
+    <ProjectVersion>2.0</ProjectVersion>
+    <DockerTargetOS>Linux</DockerTargetOS>
+    <DockerLaunchAction>LaunchBrowser</DockerLaunchAction>
+    <DockerServiceUrl>http://localhost:{ServicePort}</DockerServiceUrl>
+    <DockerServiceName>testwebapplication</DockerServiceName>
+  </PropertyGroup>
+  <ItemGroup>
+    <None Include="docker-compose.yml" />
+  </ItemGroup>
+</Project>

TestAssets/TestProjects/docker-compose/docker-compose.yml

@@ -0,0 +1,8 @@
+version: '3.0'
+
+services:
+  testwebapplication:
+    image: testwebapplication
+    build:
+      context: .
+      dockerfile: testwebapplication/Dockerfile