Add workflow to build osm-processor img in GHCR

[spwoodcock]

To be merged after #340

Issue

• Currently the osm-processor img is built locally and I assume published on Dockerhub manually.

Solution

• Add a Github workflow to build the image from this repo and publish.

[Rub21]

@batpad, do you think it is a good idea to move Docker images to GHCR? As Sam mentioned, osm-processor was uploaded manually to Docker Hub, but now I do not have access, and it seems we should find a better way to handle the images.