Support for ubuntu base images (and other platforms)

[MatthiasHinz]

We may want to be enable to support more platforms, see also rsysreqs's platform list at https://github.com/r-hub/sysreqsdb/tree/master/platforms. Issue #32 shows that we won't have too many adjustments to do.

At the moment, for using up-to-date versions of gdal users can either use the rocker/geospatial(debian-based) or install gdal from source. Using ubuntu, they could install it using the UbuntuGIS ppa. (see https://wiki.ubuntu.com/UbuntuGIS). Current versions of R can also be installed via apt (https://cran.r-project.org/bin/linux/ubuntu/). See also see https://github.com/edzer/sfr/blob/master/.travis.yml

However, for the purpose of long-term archiving, we should rather consider repositories with long-term, version-stable support.

To implement this, enhance the method .create_run_install with mappings from system requirements to system commands. See also R/defaults.R.

[nuest]

On platforms, see also https://github.com/r-hub/sysreqsdb/tree/master/platforms

[nuest]

It would be great if we could automatically identify the platform based on the (hierarchy of) Dockerfiles, see also #78