Clang docker container for TMIV CI

A docker container with modern CMake and clang for TMIV's continuous integration. Built on top of cbachhuber/clang, it additionally offers

• build caching with ccache. Inside the docker container, folder /.ccache is used for caching. Mount that one with read&write rights to a persistent folder on your machine to achieve build caching between container invocations.
• TMIV's CMake dependencies in folder /dependencies:
  • Catch2
  • fmt
  • HM
  • VVdeC
  • VVenC

Tags and corresponding docker files:

• 1: 1.Dockerfile/ contains clang 11
• 2: 2.Dockerfile/ contains clang 12