Allow node to bind to port 80 as non-root

Shorten docker build times by copying the package.json separately.

Install dependencies with npm ci

Update node to the new LTS: 14

Switch from the unmaintained node-static to serve-static.

* Do not install on run

The docker image should contains already resolved dependencies.

No need for a start.sh, you could just run node directly.

* Simplify Dockerfile