Add note to docker container.

README.md

@@ -48,6 +48,12 @@ docker build -t dot .
 docker run -it dot bash
 ```
 
+Note that some features may not work properly in the container, like system
+clipboard integration with vim or certain font icons used in themes. Still, the
+base configurations work, and using the container as a clean environment is
+useful for testing things such as installation instructions for a personal
+project.
+
 ### Configurations
 
 #### Utilities