Faciliate selection of an explicitly desired version of containerlab executable.

[fhaftmann]

The documentation explicitly states that mini-lab requires a dedicated version of containerlab.

In order not to break an existing environment, we need a possibility to use an explicitly selected version of containerlab.

Since containerlab is run through sudo, modifying PATH is not enough. Hence we need a mechanism to explicitly select containerlab by an explicit file path.

[fhaftmann]

The CI run https://github.com/metal-stack/mini-lab/actions/runs/3393599973/jobs/5641212215 fails with

Run docker/login-action@v2
  with:
    ecr: auto
    logout: true
Error: Username and password required

Are there any special preconditions to be met?

[fhaftmann]

Withdrawn: a better idea is in the pipeline.