Docker build order, change in R signing

[jpwiedekopf]

This PR is meant to do two things:

• The build order within the Dockerfile is changed so that the APT packages for R, and the R packages, are copied into the container and installed prior to the SmICS source code. This way, changes in the .NET source code don't trigger full rebuilds of the container, as the R installation will be cached already. Since installing R takes a rather long time, this increases build times significantly for me while debugging SmICS.
• Also, it changes the certificate required for installing R, as described in Change in R signing #88 . This PR replaces Change in signing R Apt repo #86 (for the version without auth); for the auth version I'll create a new PR. I'll also close Change in signing R Apt repo #86 and Change in signing R Apt repo (auth version) #87.