More Linux images

We have added three new Linux images with Jeroen:

• linux-x86_64-centos6-epel
• linux-x86_64-centos6-epel-rdt
• linux-x86_64-rocker-gcc-san

The last one runs the check with address sanitizer turned on. Unfortunately the ASAN/UBSAN output is suppressed currently,
so we still need to work on this. :(

Some lessons learned, and improvements:

• The images must have curl, this is there on the CentOS6 images by default, but not on the Rocker images.
• The images must have xvfb as well.
• We are running xvfb now with a 1024x768 screen, and 24 bits color depth. This eliminates some R problems, that come up when not enough colors are available.
• The images may set the RBINARY environment variable. The ASAN image uses this to start the ASAN-augmented version of R, which is called RD on the image.
• Jenkins now starts bash in the Docker container with -l, to make it a login shell. This allows running /etc/bashrc in the image. The epel-rdt image runs the RDT config file this way.

System requirements improvements

We have also added a bunch of new system requirements.