Build a Docker image for the SP3 branch

[lslezak]

• This is similar to the YaST approach - define separate Dockerfile for each image
• For now it still uses Tumbleweed internally as the 42.3 Docker images are not available yet (this should be fixed after the release)
• The SP3 Dockerfile is based on the original Docker image, only the repository setup has been changed and removed the libyui-gtk-devel package because it is missing in the YaST:SLE-12:SP3 repository (it is missing in SLE12-SP3 anyway, so not a problem)
• For SP3 we reuse the YaST SP3 repository which already includes the libyui SP3 packages (less maintenance), no need for a separate libyui:SP3 subproject
• Run zypper dup in the TW image (just like the YaST images)
• Updated README.md

[mvidner]

Thanks!