Add verification to `start.sh` to ensure the given YAML file exists.
Check also if the files listed in the $YAML file exist.
Stop `start.sh` with an error code when the file contains errors.

The way logs were handled did not play nice with docker log viewing
utilities.
Logs are no longer redirected from stdout.

The Dockerfile now automatically run the `start.sh` script on startup by
default.

The `$NB_WORKERS` env variable now allows to customize the number of workers
created.

`start.sh` now sleeps infinitely at the end of its execution (to avoid stopping
the container at the end of its execution).

Dockerfile tries to create an already existing symbolic link from
python2.7 to python (such link is created by default when installing
python package).

This creates some noise in the logs.

Remove useless instruction

Kaldi now requires python2 *and* python3

Add python3 to the list of dependencies, as it is now required by
Kaldi `check_dependencies.sh` script.

This change was introduced in Kaldi at:
https://github.com/kaldi-asr/kaldi/commit/8ad898c2b8a506d10b9c2fb31949a148aed6844d

It will use WORKDIR instead of cd /opt

Dockerfile for end to end example.

Information about RAM. Thanks to ReketeBravo.

Fix some typos in README