The HEALTHCHECK instruction, which is a fairly new addition to the Dockerfile, is used to define the command to run inside a container to test the container's application health. When a container has a HEALTHCHECK, it gets a special status variable. Initially, that variable will be set to starting. Any time a HEALTHCHECK is performed successfully, the status will be set to healthy. When a HEALTHCHECK is performed and fails, the fail count value will be incremented and then checked against a retries value. If the fail count equals or exceeds the retries value, the status is set to unhealthy. The syntax of the HEALTHCHECK instruction is as follows:
# HEALTHCHECK instruction syntax
HEALTHCHECK [OPTIONS] CMD command (check container health by running a command inside the container)
HEALTHCHECK NONE (disable any HEALTHCHECK inherited from the base image...
