Improve container build reproducibility and efficiency

[chgl]

The Dockerfile does not follow common practices making it not reproducible by a) using mutable tags for the build base image (lts), and b) by not using the lockfile to restore packages. Further, the COPY . . instructions works against layer caching.

See

• https://snyk.io/blog/10-best-practices-to-containerize-nodejs-web-applications-with-docker/
• https://www.docker.com/blog/9-tips-for-containerizing-your-node-js-application/
• https://nodejs.org/en/docs/guides/nodejs-docker-webapp

[juliangruendner]

@EmteZogaf please check and adjust accordingly.