pristine stdout

[briceburg]

In 7e4f37c we direct most commands to stderr to keep pristine outupt; e.g.

echo "foo" | ./main.sh run sed 's/foo/bar/' > a.txt

should result in "bar" only in a.txt (no docker build messages, etc.).

Perhaps a better strategy is to link fd3 to stderr, redirect stdout to fd3 in main.sh (which ought to cover all output), and then explicitly write to stdout in the runtime, ls, and image routines.

The worry is that commit above doesn't handle all cases.