This image is meant to be run in a Kubernetes cluster. It runs a haproxy instance that's listening on some TCP ports and load balancing incoming HTTP connections to a bunch of nodes in the cluster.
Configuration works mainly via the RUN parameters of the container (which runs a wrapper script which runs haproxy). You'll usually change those to suit your needs.
Example: The parameters: ["-t", "^node", "-p", "80:30000"] start the proxy
listening on port 80, load balancing all incoming requests to port 30000
on all nodes whose names match the pattern ^node.
The wrapper will notice (via the K8S API) when the set of nodes changes and adjust the proxy accordingly.