Add reason why we use VMs

slides/intro/Training_Environment.md

@@ -38,6 +38,26 @@ individual Docker VM.*
 
 ---
 
+## Why don't we run Docker locally?
+
+- We are going to download container images and distribution packages.
+
+- This could put a bit of stress on the local WiFi and slow us down.
+
+- Instead, we use a remote VM that has a good connectivity
+
+- In some rare cases, installing Docker locally is challenging:
+
+  - no administrator/root access (computer managed by strict corp IT)
+
+  - 32-bit CPU or OS
+
+  - old OS version (e.g. CentOS 6, OSX pre-Yosemite, Windows 7)
+
+- It's better to spend time learning containers than fiddling with the installer!
+
+---
+
 ## Connecting to your Virtual Machine
 
 You need an SSH client.