Update Guide.md

Some improvements to the section in "Fun features" that describes docker integration.

Author: wjdhamilton

doc/GUIDE.md

@@ -2030,6 +2030,23 @@ image:
 and then run `stack image container` and then `docker images` to list
 the images.
 
+Note that the executable will be built in the development environment 
+and copied to the container, so the dev OS must match that of the 
+container OS. This is easily accomplished using [Docker integration](docker_integration.md),
+under which the exe emitted by `stack build` will be built on the 
+Docker container, not the local OS. 
+
+The executable will be stored under `/usr/local/bin/<your-project>-exe`
+in the running container.
+
+If you want the container to run the executable immediately on startup
+then set an entrypoint as follows:
+
+```yaml
+entrypoints:
+    - <your-project>-exe
+```
+
 ### Nix
 
 stack provides an integration with [Nix](http://nixos.org/nix),