If your Uptime Kuma reports DOWN of your service, sometimes you would like to know it is a bug of Uptime Kuma or your own Docker network issue.
Go into your container's bash.
docker exec -it uptime-kuma bash
Then you can test with these commands for example:
curl https://google.com
ping google.com
If running a ping results in a DNS-related error, try configuring your container to run using the host's network. To do this, run the docker run command with the --network="host" flag. For example:
docker run -d --restart=always -p 3001:3001 -v uptime-kuma:/app/data --name uptime-kuma --network="host" louislam/uptime-kuma:1
More information here: https://docs.docker.com/engine/reference/run/#network-host