containertool should honour the `SOURCE_DATE_EPOCH` environment variable

`SOURCE_DATE_EPOCH` is a standard environment variable used to [override timestamps for reproducible builds](https://reproducible-builds.org/docs/source-date-epoch/). As of #38, `containertool` sets image manifest timestamps to the epoch; it already used this value for file timestamps. It should continue to use this value as a default, but should use the value of `SOURCE_DATE_EPOCH` if it is set.

The `build-container-image` plugin will also have to pass this environment variable through to `containertool`.

Start by locating the containertool code that currently chooses timestamps and the build-container-image plugin entry point. Trace how the existing epoch value reaches both components, then verify that SOURCE_DATE_EPOCH is passed through and overrides the default while unset behavior remains unchanged.

Written by the indexing model from the issue text.