README.md: edited steps to only build software env in Docker image

Until now, while the series of steps mentioned in 'README.md' were
complete, they had some implicit thing in them that made it a little hard
to run as a checklist (the commands to do some basic things weren't
included). Also, it was recommending to run a long 'docker run ...'
command, which wasn't too user friendly.

With this commit, the series of steps is now a complete checklist,
containing every step. Also, the checklist now recommends putting the long
'docker run' command inside a script called 'docker-run' that will also do
a 'sudo' internally (thus making things very easy for a first-time user).

Also, since the 'docker-run' script contains host OS-specific directory
names, it should not be under control, so it has been added to the
'.gitignore' file in case users decide to keep this same name (which is
recommended).

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