Allow container environment override

[mattBrzezinski]

We often override enviroment variables.
In particular

    --container-overrides environment="[{name=RUN_SCRIPT,value=$backrun_script_s3path}]"

but runbach does not currently take an enviroment kwarg (it takes kwarg for other things we can override)

AWSDocs: https://docs.aws.amazon.com/batch/latest/APIReference/API_ContainerOverrides.html#Batch-Type-ContainerOverrides-environment