dockerfile-mode: add 'dockerfile-image-name' local file variable

[pataquets]

As per Readme for dockerfile-mode. Notice I'm not using obsolete docker-image-name, but dockerfile-image-name. See spotify/dockerfile-mode#68.

[AndreaCrotti]

The key is quite long @pataquets , I think to make it a snippet it should save some typing, but atm it doesn't make much difference typing the whole thing out, otherwise I think it's fine.