ADD: node-alpine container for cli tool

[kaczmarj]

This pull request adds a Dockerfile for the project. I wanted to use this tool but did not have node installed, so I created a container and used the CLI through Docker.

Build the Docker image:

cd geojson-merge  # enter project root directory
docker build --tag geojsonmerge .

Use the image:

docker run --rm -v /path/to/files:/work geojsonmerge a.geo.json b.geo.json > merged.geo.json