Q.Container pepper growing
What fertilizer to use growing peppers in containers?

Use a slow release fertilizer that is formulated for vegetables.
You can also use Epsom salt to fertilizer your plants.
I use 2 Tablespoons per week watered in.
Here are some links with more information.
https://www.gardeningknowhow.com/edible/vegetables/pepper/growing-peppers.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/epsom-salt-gardening.htm
https://www.gardeningknowhow.com/edible/vegetables/pepper/growing-peppers-container.htm