Fix docker image generation in Dockerfile

[gissehel]

Closes: #4

What are the changes and their implications?

• There is a new Dockerfile
• docker-start.sh has been modified to reflect the changes
• The Dockerfile doesn't use working copy, but use source from https://github.com/droppy-js/droppy
• The Makefile is able to generate Dockerfile-dev which works like Dockerfile but for working copy

The resulting image has been tested with parametrized UID and GID, and it works as expected.

Checklist

• Changes covered by tests (tests added if needed)
• PR submitted to droppy-js.com for any user facing changes

[markhughes]

Thanks for adjusting that! Appreciate the help!