chore: exclude NuGet.Config from docker build context

NuGet.Config sets globalPackagesFolder to %USERPROFILE% for Windows.
Docker Linux builds must not inherit this - they use /root/.nuget/packages by default.

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.dockerignore

@@ -5,4 +5,5 @@
 **/obj
 **/*.user
 **/*.suo
-**/TestResults
+**/TestResults
+NuGet.Config