This commit adds a HEALTHCHECK instruction in Dockerfile.
The HEALTHCHECK instruction calls /docker/fetchstatus.sh
which in turn makes a HTTP request to
http://localhost:8000/api/v3/status/
This endpoint makes a test database query and returns non-200
response if the query fails. So, in short, if the Healthchecks
container for any reason is unable to query database, `docker ps`
will now show the container as "unhealthy".
cc: #1045
