add option for plain http registries

[kyrella]

I wanted to use my private registry without the hassle of setting up certificates, so I had to find a way to make HTTP work. Since I couldn't find a skip verify or insecure registry option like in containerd or nerdctl, I ended up digging into the source code. Go is still on my todo list, but the solution does the job. The use_plain_http option, found under task configuration, now allows registries to use plain HTTP.

task "name" {
   ...
   config {
     image = "private:latest"
     use_plain_http = true
     ...
   }
}