support iteration to generate steps from sequence

[joprice]

It would be convenient to be able to iterate over a sequence to generate commands, for example

for src, dest in [ "file-1", "b"; "file-2", "b" ] do
                  yield! dockerfile { copy src dest }

Not sure if there's a more ideal way to express that, but this seems to work with this implementation in the builder:

 member this.For(items, f) : DockerfileSpec =
              let mutable result = { Instructions = [] }
              for item in items do
                  result <- this.Combine(result, f item)
              result