Consider switching to open containers labels

## Purpose

We are currently working on adding labels to containers (see PR #2). But it appears that label-schema.org is already obsolete. We need to switch to open containers schema.

Sources:

* https://www.opencontainers.org
* https://rehansaeed.com/docker-labels-depth/

## Proposal

Switch labels to the most up-to-date schema (OCI).
