Custom images behave as on Docker in three more places

A file the image puts in /tmp is there when a sandbox starts; a string command on an image without bash, such as Alpine or BusyBox, runs under /bin/sh; and 0.0.0.0 and the sandbox's own addresses skip the proxy, so a server you start in the sandbox answers from inside it. Existing images get this at their next cold start, with no rebuild. See images.