change FROM registry.fedoraproject.org/fedora:26 to Boltron

[mkocka]

Isuggest to change first line to:
FROM modularitycontainers/boltron-preview:latest

[TomasTomecek]

@hodovska already opened a pull request to address this: https://github.com/container-images/container-image-template/pull/26

the question is whether we want to have a single fedora image based on boltron, or we also want to have a flavor using standard fedora base image.