Make --submission-service optional

[dra27]

Split off from #65, as it's not related.

[avsm]

Isn't it better to make this an option type and to do a local docker build instead of Current_ocluster.Raw.build_and_push (in src/pipeline.ml)? Making a fake uri like this will just make the local pipeline fail.

[dra27]

In this instance, the pipeline failing is the "desired" outcome - I was seeking to update the tutorial where what you're after is simply the graph of what will be built (everything up to "However, before going further you will need to configure the builders." in the README)

[talex5]

Alternative suggestion: provide a docker-compose.yml that brings up the base image builder and a local OCluster scheduler. The pipeline can connect to that without showing any errors, but the builds will just wait because there are no workers. But the user can later connect their own workers to it if they want to try building.

[talex5]

I've added a docker-compose file in #82 as suggested. It should now be possible to see the graph and also to run builds easily.