Re-enable TCC for GHA CI

[npepinpe]

Description

Since moving to our own self hosted runners with a DinD sibling, the actions controller adds an environment variable DOCKER_HOST which overrides the TCC agent settings, resulting in TCC being completely unused.

Possible solutions:

• Unset the DOCKER_HOST environment variable in the job/workflow directly.
• Disable the local DinD sibling on the actions runner directly.

[npepinpe]

We need the local DinD sibling right now to run a local registry in order to use camunda/zeebe:current-test with TCC. So the simplest solution is to unset the DOCKER_HOST environment variable for the step where we run the tests.

I'd like to time box another approach however, which is using Testcontainers to build the Docker image. This may also solve #7249. I'll timebox 3 hours tops (so until lunch), let's see.

[npepinpe]

It looks like we can't use our existing Dockerfile with the built-in ImageFromDockerFile from TC due to the following issue: testcontainers/testcontainers-java#3238

[npepinpe]

OK, so I think the final proposal would be to simply merge both containers, that way DOCKER_HOST is not defined anywhere, I think. I'll try this out and see what happens.

See https://github.com/actions-runner-controller/actions-runner-controller/blob/master/docs/detailed-docs.md#runner-with-dind

[npepinpe]

Seems to work well enough: https://github.com/camunda/zeebe/actions/runs/3371462996/jobs/5593807780

I would propose that we do this. It requires a few tweaks to the runner spec, but nothing to be done from the Zeebe side.