Dockerfile should build image from source

[i-ky]

Dockerfile which was added in #69 and #70 installs swagger-merger NPM package: https://github.com/WindomZ/swagger-merger/blob/860e4ea58b490444c15a2888e1c5fa9ef32f132c/Dockerfile#L3

It is probably the most efficient way to build a Docker image around swagger-merger, but it leaves a lot to be desired. For example, how would someone build a swagger-merger Docker image with the latest features from GitHub repo, from a branch, from a fork, with changes from someone's local working directory?

There should be a Dockerfile that allows building image from source code checkout instead of relying on NPM.