Dockerfile should build image from source

[i-ky]

Dockerfile which was added in #69 and #70 installs swagger-merger NPM package:

swagger-merger/Dockerfile

Line 3 in 860e4ea

RUN npm install -g swagger-merger watch

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.

[dhyanio]

Use yaml-merger https://github.com/dhyanio/yaml-merger fast and performant swagger-merger