What does the "(healthy)" string in STATUS column stands for?
user@user:~# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
X X X X Up 20 hours X X
X X X X Up 21 hours (healthy) X X
That's the result of the HEALTHCHECK instruction. That instruciton runs a command inside the container every 30 seconds. If the command succeeds, the container is marked healthy. If it fails too many times, it's marked unhealthy.
You can set the interval, timeout, number of retries, and start delay.
The following instruction for a Dockerfile, for example, will check that your container responds to HTTP every 5 minutes with a timeout of 3 seconds.
HEALTHCHECK --interval=5m --timeout=3s \
CMD curl -f http://localhost/ || exit 1
You get a health_status event when the health status changes. You can follow those and others with docker events.