feat: add support for non-web healthchecks via app.json

This change converts existing CHECKS files into a healthchecks key that is understood by 'docker-container-healthchecker'. This tool supports a number of different types of container healthchecks - command, http, uptime - and can perform healthchecks against non-web processes.

The use of the old CHECKS file is now deprecated, and will be removed in the next minor version. Users can use the 'docker-container-healthchecker' to convert existing CHECKS files to the new format automatically.

Closes #2760

docs/processes/process-management.md

@@ -140,7 +140,6 @@ For initial app deploys, Dokku will default to starting a single `web` process f
 
 There are also a few other exceptions for the `web` process.
 
-- Custom checks defined by a `CHECKS` file only apply to the `web` process type.
 - By default, the built-in nginx proxy implementation only proxies the `web` process (others may be handled via a custom `nginx.conf.sigil`).
   - See the [nginx request proxying documentation](/docs/networking/proxies/nginx.md#request-proxying) for more information on how nginx handles proxied requests.
 - Only the `web` process may be bound to an external port.