add support for set DOKKU_IMAGE per app
add support for set DOKKU_IMAGE per app

fixes #2745

alexquick/dokku,progrium/dokku,elia/dokku,jimjag/dokku,jimjag/dokku,jimjag/dokku,jimjag/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,elia/dokku,progrium/dokku,dokku/dokku,alexquick/dokku,dokku/dokku,jimjag/dokku,progrium/dokku,dokku/dokku,progrium/dokku,elia/dokku,alexquick/dokku,alexquick/dokku,alexquick/dokku,claytonbrown/dokku,dokku/dokku,alexquick/dokku,progrium/dokku,progrium/dokku,claytonbrown/dokku,alexquick/dokku,elia/dokku,dokku/dokku,claytonbrown/dokku,jimjag/dokku,dokku/dokku,dokku/dokku,jimjag/dokku,progrium/dokku,elia/dokku,elia/dokku,claytonbrown/dokku,jimjag/dokku,elia/dokku,alexquick/dokku,claytonbrown/dokku,elia/dokku