`containertool` should read settings from a configuration file

`containertool` has several required and optional arguments: `--repository` is required; other flags such as `--from` are optional. Some of these can be set by environment variables.

It should be possible to set default options for these flags in a configuration file in the project directory. Corresponding environment variables and command line flags should override the values in the configuration file.

Start by locating containertool's argument parsing and the existing environment-variable handling. Determine how a project-directory configuration file should represent required and optional flags, then verify that command-line arguments and corresponding environment variables override file values. Done means defaults can be loaded from the file while preserving the documented precedence.

Written by the indexing model from the issue text.