They blow in on the breeze when you open the door.
Sorry I don't have any tips, just wanted to say I feel for you. The first time I grew strawberries I had no idea what I was doing, was new to gardening. I grabbed some bare root ones (Quinalt) at the local big box store, stuck them in the ground about a foot apart in all directions in a bed about 10x10', mulched them with, guess what, straw(!) and just watered them every few days. Don't think I even fertilized them. ( I had heavy, but gritty clay soil) and had huge heath plants and ton so berries. I had them bed for about 5 years until they petered out.
Since then I know more but have had bad luck with them. I grow in pots now like you for various reasons. I've tried to give them what all the info says to, but I have that same coloration on the leaves and more than half the flowers don't make berries and the ones that do are small and bitter. They get lots of sun and are well watered.
I'm hoping you get some good answers, I'll be very interested in this thread. Good Luck!