Issues with the benchmarking instructions on the Wiki

[volyrique]

The getting started guide for benchmarking on the Wiki is missing some details or contains outdated data:

• The final docker run command is missing a parameter:
  -e USER_ID=$(id -u)
• there are no instructions how to build the techempower/tfb image, i.e. by running:
  docker build --build-arg GROUP_ID=$(id -g) --build-arg USER_ID=$(id -u) -t techempower/tfb .
  (I think that simply executing ./tfb --help would work too)
• the operating system is no longer Ubuntu Server 16.04 LTS 64-bit

Veteran contributors to the project are aware of these steps, of course, but beginners might have difficulties figuring them out.

[NateBrady23]

Thanks @volyrique!