Debian/oldstable docker container for packaging (#942)

This adds dockerized build stances to .gitlab-ci.yml. They will run only in scheduled builds (i.e. once per day at night) or upon special request (through secret GUI button). They shall ensure that at any time we are ready to generate binary installers.

This resolves #942 (closed).

devtools/deploy/windows/setx64vcvars.ps1

-# Set x64-Related MSVC Variables
-
-# To source this script, first executing scripts must be allowed by changing the execution policy.
-# Run PowerShell as an administrator and enter:
-# `Set-ExecutionPolicy Bypass`
-# See <https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_execution_policies>
-# Then, source this script, in _any_ PowerShell session via:
-# `. <script-name> [MSVC|Clang]`
-# See <https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_scripts?#script-scope-and-dot-sourcing>
-
-param(
-    [String] $Compiler = "MSVC"
-)
-
-if ($Compiler -ne "MSVC" -and $Compiler -ne "Clang") {
-    Write-Error "Unknown compiler '$Compiler'; must be MSVC or Clang"
-    Exit -1
-}
-
-Write-Host "======================================="
-Write-Host "Setting up environment variables..."
-
-# Visual Studio path <https://github.com/microsoft/vswhere/wiki/Find-VC>
-$vsPath = &"${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe" -latest -products * -requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 -property installationpath
-
-Write-Host "Microsoft Visual Studio path = '$vsPath'"
-
-# Use module `Microsoft.VisualStudio.DevShell.dll`
-Import-Module (Get-ChildItem $vsPath -Recurse -File -Filter Microsoft.VisualStudio.DevShell.dll).FullName
-
-Enter-VsDevShell -VsInstallPath $vsPath -SkipAutomaticLocation -DevCmdArguments '-arch=x64'
-
-# NOTE: `-DevCmdArguments` are arguments to `vsdevcmd.bat`
-
-# Select compiler
-if ($Compiler -eq "MSVC") {
-    $_Compiler = "MSVC"
-    Set-Item -Path "env:CC" -Value "cl.exe"
-    Set-Item -Path "env:CXX" -Value "cl.exe"
-}
-elseif ($Compiler -eq "Clang") {
-    $_Compiler = "Clang"
-    Set-Item -Path "env:CC" -Value "clang-cl.exe"
-    Set-Item -Path "env:CXX" -Value "clang-cl.exe"
-}
-
-Write-Host "Selecting $_Compiler as C/C++ compiler."
-Write-Host "======================================="