Add sidecar variants

[michaelsauter]

Closes #135.

FYI @gerardcl @renedupont I think I mentioned this to you at some point, and both of you have a use case IIRC. Would love to hear your feedback. BTW: if you only want to see the actual changes, look at f6011ab only. The rest is noise.

To pre-empt the question "What happens if I need more than one sidecar, or if I need to modify it?":

With this solution, there are two options:

1. Continue to use the officially provided one-sidecar-task. If you need to run two services, you could create a custom image with both services. Obviously not ideal. Also, it breaks down if you need to modify env vars or resources.
2. Copy the cluster task and create your own local task, which you then can modify.

If you have other ideas, happy to explore!

[gerardcl]

hi! I guess until we don't jump into testing/using it we won't be 100% sure how we can "tweak" it ;)

Our use case is basically spinning up a redis and a postgres aside of the jenkins agent. So tests from the different components can call them (since we prefer avoiding mocking stores as much as possible).

I guess based on f6011ab#diff-8c3ef8a2a45c76e026448030a1ad3389d7676f0664ee84e0f9c8c17cd7f6fcc0R76 we could just keep adding sidecars, right?

Maybe in the future we can come up on how to provide a list of sidecar images and make it automagically integrated and spinned up?

[michaelsauter]

The limitation here is that the sidecars are defined by the task author, not the pipeline author. Tekton does not allow you as a consumer of tasks to specify sidecars. If it did, we would not need "sidecar variants" in the first place. I opened an issue with Tekton (tektoncd/pipeline#4235) but so far no response :(

[michaelsauter]

This also means that if you want to use ods-build-python-with-sidecar, you need to provide an image that includes both Redis and Postgres. Otherwise you need to roll your own task (copying ods-build-python-with-sidecar to your own namespace and adding another sidecar).

[gerardcl]

exactly! hence having the ods-build-python-with-sidecar as an example would be a good way to go! LGTM