Allow configuring container creation timestamp in plugins

[TadCordle]

instead of just whether or not to use current timestamp. We could change the useCurrentTimestamp parameter to take either a boolean, or an integer number of milliseconds since epoch, or rename the parameter to just timestamp and take a string to determine whether to use current timestamp or a specific timestamp.

[ST-DDT]

I would like to set the docker image timestamp to the time of the git commit that was used to build the underlying java classes. IIRC the docker image list has a created date, that shows the age of the image. Falsifying this image date results in a loss of useful information especially if "old images" should be garbage collected.

For plugins it might be useful to provide a timestamp + a format string to allow optimal flexibility when choosing the timestamp source.

[chanseokoh]

1.5.0 released with the feature to configure the container creation time. See the jib.container.creationTime doc for Maven/Gradle.

For configuring file modification times, see #1608 (comment).

@ST-DDT