Add DNS-server issue explanation (#1917)

A lot of people seem to be searching for answers when this happens, and the obvious answer is not easily found. On my QNAP nas I had to specify this to allow the container to resolve adresses on the internet succesfully,

README.md

@@ -18,6 +18,7 @@ docker run -p 8080:8080 -p 50000:50000 --restart=on-failure jenkins/jenkins:lts-
 ```
 
 NOTE: read the section [_Connecting agents_](#connecting-agents) below for the role of the `50000` port mapping.
+NOTE: read the section [_DNS Configuration_](#dns-configuration) in case you see the message "This Jenkins instance appears to be offline." 
 
 This will store the workspace in `/var/jenkins_home`.
 All Jenkins data lives in there - including plugins and configuration.
@@ -114,6 +115,15 @@ If you want to install Jenkins behind a reverse proxy with a prefix, example: my
 - [Apache](https://www.jenkins.io/doc/book/system-administration/reverse-proxy-configuration-apache/)
 - [Nginx](https://www.jenkins.io/doc/book/system-administration/reverse-proxy-configuration-nginx/)
 
+## DNS configuration
+
+If the message "This Jenkins instance appears to be offline." appears on first startup, and the container logs show lines such as `java.net.UnknownHostException: updates.jenkins.io`, your container may be having issues with resolving DNS names.
+
+To potentially solve the issue, start the container specifying a dns server (for example Cloudflare's 1.1.1.1 or Google's 8.8.8.8, or any other DNS server):
+```
+docker run -p 8080:8080 -p 50000:50000 --restart=on-failure --dns 1.1.1.1 --dns 8.8.8.8 jenkins/jenkins:lts-jdk17
+```
+
 ## Passing Jenkins launcher parameters
 
 Arguments you pass to docker running the Jenkins image are passed to jenkins launcher, so for example you can run:
@@ -312,6 +322,17 @@ This will pull the necessary images from Docker Hub if they are not already pres
 
 You can then access the Jenkins web interface on `http://localhost:8080` on your host system to configure and manage your Jenkins instance (where `localhost` points to the published port by your Docker Engine).
 
+NOTE: read the section [_DNS Configuration_](#dns-configuration) in case you see the message "This Jenkins instance appears to be offline." In that case add the dns configuration to the yaml:
+```yaml
+services:
+  jenkins:
+# ... other config
+    dns:
+      - 1.1.1.1
+      - 8.8.8.8
+# ... other config
+```
+
 ### Updating plugins file
 
 The [plugin-installation-manager-tool](https://github.com/jenkinsci/plugin-installation-manager-tool) supports updating the plugin file for you.