Change Docker base image to slim

[zipiju]

Slim shaves about 400MB off the container when installed. It is always better to have smaller images, especially when installing these at the edge on routers and such.

[juliangruber]

+1 if CI passes

[bajtos]

For context: we moved from Alpine to Full distro because Zinnia does not support Alpine yet and Saturn L2 module was not able to run on the Slim version.

See #92

We cannot use node:18-alpine because Zinnia does not support that yet.

I tried to use node:18-slim, but Saturn L2 was not able to start in that system.
The Node.js project recommends to use node:18, so we should be fine. See https://github.com/nodejs/docker-node#image-variants

Now that we are no longer shipping Saturn L2, it makes a lot of sense to switch to Slim version.

Thank you for contributing this improvement, @zipiju! ❤️