adding Docker troubleshooting to README

README.md

@@ -209,6 +209,17 @@ The `dev/` folder contains a script for completing a hard reset: destroying and
 sudo ./dev/rebuild.sh
 ```
 
+## Troubleshooting Docker
+Instead of defaulting to a system DNS server, Docker will try to use a public DNS server (e.g. 8.8.8.8). If you're having issues pulling or building Docker containers with "failure to connect" errors, this is the likely problem. You can manually set the DNS server Docker should use with the `DOCKER_OPTS` field in `/etc/default/docker`. Example:
+```
+DOCKER_OPTS="--dns 192.168.50.1 --dns 1.1.1.1"
+```
+
+Be sure to restart Docker afterward:
+```
+sudo systemctl restart docker
+```
+
 # Contact
 
 Contact me at [steven@cyber5k.com](mailto:steven@cyber5k.com)