updates to documentation for Docker-based installation examples

[mmguero]

Some changes we need to do:

• check steps and update examples so that they use the latest Ubuntu LTS as the basis for the instructions
• in the Ubuntu install example, we need to split it out into two different complete end-to-end lines: one for the Git-based install, start-to-finish, and one for the tarball based install, start-to-finish; or we need to just pick one but then we need to move the steps for the other one to another place in the documentation (the differences are minimal, I think we can just keep this the way it is)

[mmguero]

reported by @SnowRideN