`containertool` should be able to set or override a container image's environment variables

[euanh]

It should be possible to set default environment variables on the command line e.g.:

containertool --env HOSTNAME=0.0.0.0 --env PORT=8080

This would set these variables in the container image metadata, with the same effect of using ENV in a Containerfile.

The --env flag should be passed through to containertool by the SwiftPM plugin.

[Joannis]

This is fixed now right?

[euanh]

Yes, I think #171 fixes this.