Make the api backwards compatible with Docker 18.06.1-ce, API version 1.38

[gesellix]

Works around:

Caused by: com.squareup.moshi.JsonDataException: Non-null value 'warnings' (JSON name 'Warnings') was null at $.Warnings
 	at com.squareup.moshi.internal.Util.unexpectedNull(Util.java:663)
 	at de.gesellix.docker.remote.api.ContainerCreateResponseJsonAdapter.fromJson(ContainerCreateResponseJsonAdapter.kt:45)
 	at de.gesellix.docker.remote.api.ContainerCreateResponseJsonAdapter.fromJson(ContainerCreateResponseJsonAdapter.kt:22)
 	at com.squareup.moshi.internal.NullSafeJsonAdapter.fromJson(NullSafeJsonAdapter.java:41)
 	at com.squareup.moshi.JsonAdapter.fromJson(JsonAdapter.java:70)
 	at de.gesellix.docker.remote.api.client.ContainerApi.containerCreate(ContainerApi.kt:2093)