Update documentation on how to enable the Data Plane API in 3.1 and above

[WIStudent]

HAProxy 3.1 deprecated the program section, so to enable the Data Plane API as described in the readme you need to set the expose-deprecated-directives global option.

global
   expose-deprecated-directives

program api
   command /usr/bin/dataplaneapi --host 0.0.0.0 --port 5555 --haproxy-bin /usr/sbin/haproxy --config-file /usr/local/etc/haproxy/haproxy.cfg --reload-cmd "kill -SIGUSR2 1" --restart-cmd "kill -SIGUSR2 1" --reload-delay 5 --userlist haproxy-dataplaneapi
   no option start-on-reload

The deprecation announcement also states that the program section will be removed in HAProxy 3.3. Please update the readme with instructions on how to use the Data Plane API in this image without the program section.

[dkorunic]

This is correct, using program was pretty much never the best choice compared to for instance s6 overlay. However we didn't want to add s6 overlay by default due to many people using our images for their own projects, and enforcing a single solution is definitely not an ideal approach. I'll detail how to approach this and install s6 overlay, together with examples of configs, similar to how we do it in Ingress Controller project.