Please update the published image

[maksym3d]

One of the last updates to the code seem to be a fix to get_hosts. However, if you just follow the instructions in https://github.com/NLKNguyen/alpine-mpich/wiki/Multi-Host-Orchestration without building the images first and just do ./swarm.sh up size=5 - it would seemingly pull a published 'onbuild' image from a docker registry. This image contains an old version of get_hosts that does not seem to work in my setup. The new version works fine, but it took some time to figure out what's going on. Thanks!