Document concurrency issues with file-based apps (#51435)

Add note about concurrent invocations causing errors.

From dotnet/sdk#52773

Author: DamianEdwards

docs/core/sdk/file-based-apps.md

@@ -363,6 +363,9 @@ Caching improves build performance but can cause confusion when:
 - Changes to implicit build files don't trigger rebuilds.
 - Moving files to different directories doesn't invalidate cache.
 
+> [!NOTE]
+> Concurrent invocations of a file-based app (for example, running more than one instance of the same file-based app in parallel) can cause errors due to contention over the build output files. To avoid this, first build the file-based app via `dotnet build file.cs` before starting the concurrent instances via `dotnet run file.cs --no-build`.
+
 ### Workarounds
 
 - Clear cache artifacts for file-based apps by using the following command: