Add Dokku CHECKS

[natanlao]

Currently fails with

remote: -----> Attempting pre-flight checks (web.1)        
remote:        CHECKS expected result:        
remote:        http://localhost/ => "youtube-dl as a service"        
remote:        Attempt 1/5. Waiting for 5 seconds ...        
remote:  !     #=#=#                                                                                 
remote: ##O#- #                                                                               
remote: curl: (7) Failed to connect to 172.17.0.3 port 80: Connection refused        
remote:  !     Check attempt 1/5 failed.        
remote:        Attempt 2/5. Waiting for 5 seconds ...        
remote:  !     #=#=#                                                           
...