Drone: Add Windows PR pipeline (#26589)

* Drone: Add Windows PR pipeline
* CI: Add Dockerfile for WiX image

scripts/build/ci-wix/Dockerfile

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+# This has to correspond to the version the Drone runners have
+FROM mcr.microsoft.com/windows:1809
+
+WORKDIR C:\\App
+
+RUN powershell Set-ExecutionPolicy RemoteSigned -Scope CurrentUser -Force
+RUN powershell Invoke-Expression (New-Object System.Net.WebClient).DownloadString('https://get.scoop.sh')
+RUN scoop install wixtoolset@3.11.2
+RUN powershell (New-Object Net.WebClient).DownloadFile(\
+  \"https://grafana-downloads.storage.googleapis.com/ci-dependencies/nssm-2.24.zip\", \
+  \"nssm-2.24.zip\")
+RUN scoop install git@2.28.0.windows.1
+RUN scoop bucket add extras
+RUN scoop install gcloud@305.0.0
+# Installing dos2unix fails if not under PowerShell
+RUN powershell scoop install dos2unix
+
+ENTRYPOINT ["powershell"]

scripts/build/ci-wix/Magefile.go

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+//+build mage
+
+package main
+
+import (
+	"github.com/magefile/mage/mg"
+	"github.com/magefile/mage/sh"
+)
+
+const imageName = "grafana/ci-wix:0.1.1"
+
+// Build builds the Docker image.
+func Build() error {
+	if err := sh.RunV("docker", "build", "-t", imageName, "."); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Publish publishes the Docker image.
+func Publish() error {
+	mg.Deps(Build)
+	return sh.RunV("docker", "push", imageName)
+}
+
+var Default = Build

scripts/build/ci-wix/README.md

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+# WiX Docker Image
+This directory contains a Dockerfile for building a Windows based image containing
+the WiX toolkit, that we use to build Windows installers.
+
+To build the Docker image:
+
+```
+mage
+```
+
+To publish the Docker image:
+
+```
+mage publish
+```