Old house. Front porch light wiring is piped so need to know how to install an outlet box. Pic attached
The picture shows an armored conduit holding the wires. You could mount a box by removing the nut and mounting a box through a knockout at the back of the box, replacing the nut on the inside. You'd want to make sure it's rated for outdoor since it's exposed.
I'm not sure what type of base the light has and whether or not it will cover the box once you mount it or if that's important to you.
In this particular application code may allow you to attach the light directly to a mounting bar attached to the nipple. However, before you do that check codes. One of the electrical pros here might help you with that.