Clean up Docker build

Remove LOFARBeam and Dysco, and clean up.

• Removed LOFARBeam and Dysco from the build:
  • LOFARBeam doesn't build against Casacore 3.6.1 and has been superseded by EveryBeam;
  • Dysco is nowadays part of Casacore, which we now build ourselves.
• Cleaned up the Dockerfile:
  • grouped ARG statements near the top;
  • added -DTARGET_CPU argument to the cmake statement, for those packages that support it.