Added Dockerfile to create ubuntu build environment

[ns-skumarbendalam]

No description provided.

[necouchman]

@ns-skumarbendalam If you intend to contribute these changes to the project, you'll need to get a Jira account, open a Jira ticket for the changes, and update this pull request with that information. Please see:

• https://issues.apache.org/jira/browse/GUACAMOLE
• https://guacamole.apache.org/open-source/

That said, I'm curious why you feel the project needs an Ubuntu build environment? As a project we've very specifically chosen the current base images, and I think we'd need a very good reason to either add an alternative or move away from those. Adding an alternative just adds to the amount of work to maintain the environment for something that's supposed to just work out of the box without really caring what platform it's built on.