Create Dockerfile to build Jenkins Docker Slaves

Create Dockerfile to build Jenkins Docker Slaves

This Dockerfile is used for building container images to
use as Jenkins Slaves for Nordix Infra. The image is configured
and tested for running
  - Jenkins Job Builder Verify and Merge jobs using tox
  - Cloud Infra Engine ansible-lint, yamllist, and shellcheck using tox

The image built using this Dockerfile is currently being stored on Docker Hub
with the name nordixorg/infra-tools-docker-slave-ubuntu1804 which can be seen
using the link below.
  https://hub.docker.com/repository/docker/nordixorg/infra-tools-docker-slave-ubuntu1804
The images are automatically attached to Jenkins as slaves using Jenkins
Docker plugin with a name that looks like infra-ubuntu1804-<random_characters>

In order to use this image as slave for your jobs, you need to use label
infra-tools-docker-slave-ubuntu1804 so Jenkins schedules your job on this slave
which gets provisioned by Jenkins Docker plugin. Please note that it takes few
seconds before the slave becomes ready and job starts running.

Change-Id: I13c03bb9c76fedaf0222376d35f84a67a4266662