`containertool` should be able to configure image parameters such as arguments, environment etc

It should be possible to override the runtime defaults [inherited from the base image](https://github.com/apple/swift-container-plugin/blob/0c9ea97611f7344dc5408794f33df8c3c513e2a0/Sources/containertool/containertool.swift#L184).

It should be possible to set these values using command line options and environment variables.