[doc] add a README doc for otbr docker for common issues

etc/docker/README.md

@@ -0,0 +1,34 @@
+# OT BR Docker
+
+## Troubleshooting
+### rsyslog cannot start
+If `rsyslog` cannot start successfully (don't have response) during otbr docker image booting-up:
+```
++ sudo service rsyslog status                        
+ * rsyslogd is not running   
++ sudo service rsyslog start
+ * Starting enhanced syslogd rsyslogd 
+```
+
+This is caused by the high limit number of file descriptors (`LimitNOFILE`). The program of `rsyslog` will take a long time to run a for loop to close all open file descriptors when this limit is high (for example, `1073741816`).
+
+To solve this issue, add the following configuration to `/etc/docker/daemon.json`:
+```
+  "default-ulimits": {
+    "nofile": {
+      "Name": "nofile",
+        "Hard": 1024,
+        "Soft": 1024
+    },
+    "nproc": {
+      "Name": "nproc",
+        "Soft": 65536,
+        "Hard": 65536
+    }
+  }
+```
+And then reload & restart the docker service:
+```
+sudo systemctl daemon-reload
+sudo systemctl restart docker
+```