Add Docker commands

src/docker.md

@@ -58,4 +58,16 @@ docker cp path/to/file <container>:./destination/path/
 docker exec -it <container> bash
 ```
 
+## System
+```bash
+# Show disk usage.
+docker system df
+docker system df --verbose
+
+# Show active container stats.
+docker stats
+
+# View and prune images.
+docker image ls --filter dangling=true
+docker image prune
 ```

src/linux.md

@@ -179,6 +179,7 @@ htop
 
 # Memory usage.
 free
+# Human readable.
 free -h
 # Show in MBs.
 free -m