The container image should include any resources associated with the target being packaged

[euanh]

The plugin currently packages the target executable in the container image. If Package.swift declares resources for the target, these should be included as well.

This requires:

• Improving containertool's minimal tar library to package multiple files in one archive
• Adding a flag to tell containertool which resource directory to include
• Improving the plugin to pass the resources directory to containertool