Configurable Healthcheck Expose Port

[rience]

Healthcheck port (exposed one) is hard coded to 3999. This is a problem when doing parallel deployments and might result in this error

ERROR (SSHKit::Command::Failed): Exception while executing on host plus-6fc20fca3f82fa64049fec08bac4b533.svc: docker exit status: 125
--
  | docker stdout: fec19f2cf5310d4900e21ed48681ce9e896901185f656cfd1797d109af9bf251
  | docker stderr: docker: Error response from daemon: driver failed programming external connectivity on endpoint healthcheck-xxx (92d063a6607cbf7525970c123a75598dcde5dae34862ad9e436c05e9bcb82406): Bind for 0.0.0.0:3999 failed: port is already allocated.

This PR adds an option to healthcheck configuration and allow to pass exposed_port via configuration. Default value stays the same and is still 3999.

[brunoprietog]

Can you document this in kamal-site? Otherwise I will try to do it when I have some time

[rience]

PR created: basecamp/kamal-site#23