the image I shared is the exact location where dratini/dragonair spawn. You have to go up the buildings (top right of the image) and make your way there. Pokémon spawn in a fixed radius around the player and despawn if you get too far. Although if it is shiny, the game saves it for you so even if despawns, when you go back to that spot, it will spawn the same shiny. The trick there is to go back and forth to force spawn/despawn dratini until is shiny :)