It's not as hard as you might think.
In order to breed over an egg move, you will need a Female of the pokemon you are trying to breed and a Male of a pokemon that is within the same egg group as the female pokemon that also has Fire Fang in it's move pool.
Rockruff is in the "Field" Egg group, so you would need to breed her with another Male "Field" Egg group pokemon that has access to the move Fire Fang.
So for example, Growlithe is part of the "Field" Egg group and can learn Fire Fang at level 21. So you would need a Male Growlithe with the attack Fire Fang and breed it with a Female Rockruff. The offspring of those two pokemon will be Rockruff with Fire Fang. Then if you want to breed that Rockruff to get offspring that is either shiny or has better IV's just throw it in to the Day Care center with either another Rockruff or a Ditto. If you breed the Egg move Rockruff with another Rockruff, it will always work if the Rockruff with the Egg move is a male. However if you are breeding the Egg move Rockruff with a Ditto, then the gender of the Rockruff does not matter and the Egg move will always transfer over to the offspring.
Here is a list of all the pokemon that learn Fire Fang either by level up or by Egg move. If you look at each pokemon's page and look under the category that says "Egg Group", it will tell you which group(s) they belong to and that will be your identifier as to whether or not the pokemon is compatible for breeding with your Rockruff or not.
Hope this helps, once again good luck!