Multiple improvements for Dockerfile

There are multiple issues in the used Dockerfile, so I suggest a small refactor/change for fixing:

• Fix #103 (closed)
• Fix #102 (closed)
• Missing WORKDIR instruction (default working directory inside the container currently is / )
• Use COPY instead of ADD
  • local directory is used which is not archive or remote url
• Add CMD command with default --help arg
  • diffoscope invocation without params return "error: the following arguments are required: path1" with exit code 2, using default --help arg will return 0 exit code and the help text without error.
• Copy only required files during deps install to avoid cache busts.
• Remove dist-upgrade
  • docker documentation say to avoid it & it might not work at all https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#apt-get
• Merge RUN commands to reduce number of layers inside the image
• Fix "Use of apt-get update should be paired with rm -rf /var/lib/apt/lists/* in the same layer."
• Dockerfile lint tools return zero (or accepted minimum) results
  • test with https://www.dockerfilelint.com and https://hadolint.github.io/hadolint/ and https://www.fromlatest.io and https://github.com/projectatomic/dockerfile_lint
• Missing LABEL org.opencontainers.image.* entries

Will work on it in upcoming week and post a example of fixed Dockerfile as comment in this issue :)