Migrates to Pester5+ tests (#59)

#### 📖 Summary

- update Pester tests to Pester5+ compatiple layout
- switch to dotnet base imaged
  - dotnet binary required for publishung powershell modules (Publish-Module)

#### 📑 Test Plan

> 💡 Select your test plan for the code changes.

- [x] Tested via Drone.io pipeline
- [ ] Custom test
- [ ] No test plan

##### Details / Justification

<!-- Add your test details or justification for missing tests here. -->

#### 📚 Additional Notes

<!-- A place for additional detail notes. -->

Co-authored-by: OCram85 <marco.blessing@googlemail.com>
Reviewed-on: OCram85/PSCredentialStore#59

src/ModuleRoot.psm1

@@ -0,0 +1,11 @@
+#region module-definition
+
+#endregion module-definition
+Set-Variable -Name "CSVersion" -Value "2.0.0" -Option Constant -Scope 'Script' -ErrorAction Stop
+
+# Get all child items in the Script path and exclude the Deploy script (if present.)
+$Functions = Get-ChildItem -Path $PSScriptRoot\*.ps1 -Recurse | Where-Object { $_.BaseName -notmatch '.Tests' }
+
+foreach ($Item in $Functions) {
+    . $Item.FullName
+}