fix to run npm command in a local container if npm is not available

Merged Hannes Fuchs requested to merge fix-run-via-docker-container into master

This builds a image via CI and locally. The image contains required dependencies to run the npm commands build and start in the container. reveal.js uses Puppeteer, which requires chrome/chromium. Chrome/chromium is also the reason for --cap-add=SYS_ADMIN, otherwise it will crash. An option would be to use --no-sandbox, but that needs patching Puppeteer/reveal.js. Using the container registry requires a login, when building the image locally we can run ./ and only needs Docker.

Also replaces deprecated only keyword with rules in gitlab ci.

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