images tool lists images from all execs

[siddharthab]

Ideally, the images list should only consider execs that are going to run.

For example, consider:

val a = exec(image := "dummy") (out file) {" echo "a" > {{out}} "}
val b = exec(image := "ubuntu") (out file) {" echo "b" > {{out}} "}

@requires(cpu := 1)
val Main = b

This lists both dummy and ubuntu as images.

[mariusae]

This is impossible.