Do not validate variable substitutions in RUN instructions

[rcjsuen]

In #56 we added support for the ? modifier in RUN instructions. However, from a report in microsoft/vscode-docker#952 we can see that PowerShell (unsurprisingly) works quite different than Bash (and presumably other shells in the *nix world).

Hence, we should instead skip the validation of variable substitutions if they are found within a RUN instruction.

$ dockerfile-utils lint Dockerfile.1.1 
Line: 42
    Remove-Item -Force -Recurse \"${Env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\"; `
                                        ^
Error: failed to process "${Env:ProgramFiles(x86)}": unsupported modifier (P) in substitution

Line: 43
    Remove-Item -Force -Recurse ${Env:TEMP}\*; `
                                      ^
Error: failed to process "${Env:TEMP}": unsupported modifier (T) in substitution

Line: 44
    Remove-Item -Force -Recurse \"${Env:ProgramData}\Package Cache\"
                                        ^
Error: failed to process "${Env:ProgramData}": unsupported modifier (P) in substitution

# escape=`

ARG REPO=mcr.microsoft.com/dotnet/framework/runtime
FROM $REPO:4.8-windowsservercore-ltsc2016

SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue';"]

# Install NuGet CLI
ENV NUGET_VERSION 4.4.1
RUN New-Item -Type Directory $Env:ProgramFiles\NuGet; `
    Invoke-WebRequest -UseBasicParsing "https://dist.nuget.org/win-x86-commandline/v$Env:NUGET_VERSION/nuget.exe" -OutFile $Env:ProgramFiles\NuGet\nuget.exe

# Install VS Test Agent
RUN Invoke-WebRequest -UseBasicParsing "https://download.visualstudio.microsoft.com/download/pr/446c5efe-9162-41a1-b380-704c82d13afa/8c6c6f404ed99e477007f16a336f99a6/vs_testagent.exe" -OutFile vs_TestAgent.exe; `
    Start-Process vs_TestAgent.exe -ArgumentList '--quiet', '--norestart', '--nocache' -NoNewWindow -Wait; `
    Remove-Item -Force vs_TestAgent.exe; `
    # Install VS Build Tools
    Invoke-WebRequest -UseBasicParsing "https://download.visualstudio.microsoft.com/download/pr/df649173-11e9-4af2-8eb7-0eb02ba8958a/cadb5bdac41e55bb8f6a6b7c45273370/vs_buildtools.exe" -OutFile vs_BuildTools.exe; `
    # Installer won't detect DOTNET_SKIP_FIRST_TIME_EXPERIENCE if ENV is used, must use setx /M
    setx /M DOTNET_SKIP_FIRST_TIME_EXPERIENCE 1; `
    Start-Process vs_BuildTools.exe `
    -ArgumentList `
    '--add', 'Microsoft.VisualStudio.Workload.MSBuildTools', `
    '--add', 'Microsoft.VisualStudio.Workload.NetCoreBuildTools', `
    '--add', 'Microsoft.VisualStudio.Workload.ManagedDesktopBuildTools', `
    '--add', 'Microsoft.VisualStudio.Workload.VCTools', `
    '--add', 'Microsoft.VisualStudio.Workload.NodeBuildTools', `
    '--add', 'Microsoft.VisualStudio.Workload.MSBuildTools', `
    '--add', 'Microsoft.VisualStudio.Workload.WebBuildTools', `
    '--add', 'Microsoft.VisualStudio.Workload.AzureBuildTools', `
    '--add', 'Microsoft.VisualStudio.Workload.VisualStudioExtensionBuildTools', `
    '--add', 'Microsoft.Net.ComponentGroup.4.7.DeveloperTools', `
    '--add', 'Microsoft.Net.ComponentGroup.4.7.1.DeveloperTools', `
    '--add', 'Microsoft.VisualStudio.Workload.NetCoreBuildTools', `
    '--add', 'Microsoft.Component.ClickOnce.MSBuild', `
    '--add', 'Microsoft.VisualStudio.Component.VC.v141.MFC', `
    '--add', 'Microsoft.VisualStudio.Component.VC.v141.ATL', `
    '--remove', 'Microsoft.VisualStudio.Component.Windows81SDK', `
    '--quiet','--includeRecommended','--includeOptional', '--norestart', '--nocache' `
    -NoNewWindow -Wait; `
    Remove-Item -Force vs_buildtools.exe; `
    Remove-Item -Force -Recurse \"${Env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\"; `
    Remove-Item -Force -Recurse ${Env:TEMP}\*; `
    Remove-Item -Force -Recurse \"${Env:ProgramData}\Package Cache\"