docker system commands added (#1778)

* docker system commands added

* .cache directory removed

* .cache directory remove along with all unncessary files inside it

* .cache directory added to gitignore

* .cache in git ignore modified

.gitignore

@@ -4,6 +4,7 @@ _site
 /node_modules
 /vendor
 .idea/
+.cache/
 
 # Generated by 'yarn dev'
 /_includes/2017/critical/*

docker.md

@@ -183,6 +183,28 @@ docker volume prune
 
 Delete all the volumes
 
+### Systems
+
+```sh
+docker system df
+```
+
+Show docker disk usage
+
+```sh
+docker system events
+```
+
+Get real time events from the server
+
+```sh
+docker system info
+```
+
+Display system-wide information
+
+
+
 Also see
 --------