Allow setting ENVs for the cluster using NewTestDockerCluster

[gkoutsoumpakis-form3]

Is your feature request related to a problem? Please describe.
I am trying to use the docker.NewTestDockerCluster to create some test cases but, it looks like there is currently no way to set environment variables as part of the cluster initialisation. This seems restrictive as there is no way to set any AWS_* variables (or at least I haven't spotted one)

Describe the solution you'd like
Allow to pass in optionally extra variables through DockerClusterOptions

Describe alternatives you've considered
I didn't think of a better alternative, but if I missed something please let me know

Explain any additional use-cases

Additional context