Utilize Debian Snapshots

[nuest]

Debian runs a snapshots repository where we can see how the Debian apt repositories looked at a specific point in time, see http://snapshot.debian.org/.

We could use this to not only glue the R packages to a specific point in time, but also the Debian packages to a user-defined timestamp defined as part of the dockerfile method.

Some ideas for features/use cases:

• [ ] overwrite Rocker's MRAN with user-defined timestamp for MRAN
• [ ] set both MRAN and Debian Snapshot to the same day (which time of day does MRAN preserve? "Since September 17th, 2014, the checkpoint server has been taking a daily snapshot at precisely midnight UTC of the entire CRAN repository and storing it on MRAN.")
• [ ] set the Debian Snapshot based on the pre-configured MRAN timestamp

Thanks @eddelbuettel for pointing this out at useR!2017.