App Container is not validate properly

[janisz]

Trying to create following app:

{
  "id": "docker-compose-demo",
  "cmd": "echo hello world",
  "container": {}
}

results with:

HTTP/1.1 500 Internal Server Error

{"message":"docker must not be empty"}%  

Should be

 HTTP/1.1 422 

{"message":"Object is not valid","details":...

Reported in: #4945

[meichstedt]

Affects 1.4 but I don't know whether that'll land in 1.4

[unterstein]

PR @ #4989