Use containers instead of dockerbuild.sh (#23497)

Author: John Luo

.azure/pipelines/ci.yml

@@ -433,7 +433,8 @@ stages:
       jobName: Linux_musl_x64_build
       jobDisplayName: "Build: Linux Musl x64"
       agentOs: Linux
-      buildScript: ./dockerbuild.sh alpine
+      container: mcr.microsoft.com/dotnet-buildtools/prereqs:alpine-3.9-WithNode-0fc54a3-20190918214015
+      buildScript: ./build.sh
       buildArgs:
         --arch x64
         --os-name linux-musl
@@ -466,7 +467,8 @@ stages:
       jobDisplayName: "Build: Linux Musl ARM64"
       agentOs: Linux
       useHostedUbuntu: false
-      buildScript: ./dockerbuild.sh ubuntu-alpine37
+      container: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm64-alpine-20200413125008-406629a
+      buildScript: ./build.sh
       buildArgs:
         --arch arm64
         --os-name linux-musl

.azure/pipelines/jobs/default-build.yml

@@ -114,6 +114,8 @@ jobs:
           name: NetCoreInternal-Pool
           # Visual Studio Enterprise - contains some stuff, like SQL Server and IIS Express, that we use for testing
           queue: BuildPool.Server.Amd64.VS2019
+    ${{ if ne(parameters.container, '') }}:
+      container: ${{ parameters.container }}
     variables:
     - AgentOsName: ${{ parameters.agentOs }}
     - ASPNETCORE_TEST_LOG_MAXPATH: "200" # Keep test log file name length low enough for artifact zipping