Release matching the CAV'24 tool paper on CaDiCaL 2.0.

• We have now a contrib directory and for starters there our
  CadiCraig interpolator, which goes through the Tracer API.

• We moved back to use the C99 flexible array member feature in
  Clause which however is not supported by all C++ compiler
  configurations, particularly if compiling in pedantic mode.
  Therefore the configure script checks for support of flexible
  array members and also has a new --no-flexible option.

• Added Dockerfile to support docker containers.

• Added --no-status to skip printing s SATISFIABLE or s UNSATISFIABLE.
  This is useful for online proof checking.