containertool should honour the `SOURCE_DATE_EPOCH` environment variable

[euanh]

SOURCE_DATE_EPOCH is a standard environment variable used to override timestamps for reproducible builds. 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.