Build arm version of fetchit-sample-app

[cwiggs]

While following the quick start guide here: https://fetchit.readthedocs.io/en/latest/quick_start.html I wasn't able to get very far because the container in the welcome raw yaml is quay.io/fetchit/fetchit-sample-app:latest, which does not have an arm version.

It should be easy enough to build an arm version but I can't find anywhere in the repo where the dockerfile is, or any automation around how it was pushed to quay. If that info is provided I would be happy to submit a PR.

Thanks!