Fix npm error, add compose

[Alwinius]

This fixes issue #35 and makes a quick deployment easier by providing a docker-compose.yml. The instructions to add the node repository come from here https://github.com/ether/etherpad-lite#installation. However I didn't try the postgres support. If you replace the image: tvelocity/etherpad-lite line with build: . you can quickly test the Dockerfile without using the image from docker hub.

[tvelocity]

Switching to the official node image has resolved this issue, since the official node base image is now compatible.

For the docker compose changes, I would prefer them as separate commits. Generally one change per commit is the preferred way to work with git. Pull requests should also have a common theme, so pulling one change doesn't imply another change that might need to be reviewed separately.

That said it is long overdue that a few docker compose examples are included, so I will check your implementation as soon as I find the time for it.

[Alwinius]

You're right. Feel free copy as much as you want from my example.