[Meta] simplify dedicated server docker build

[Razish]

Introduce .dockerignore to ignore common build directories that would inflate the builder layer and slow down the COPY step. Switch from ubuntu to debian base image, effectively upgraded (18.04 -> bookworm/2023). Install only necessary packages, simplify apt-get invocation. cmake: don't require JPEG and PNG libraries unless we're building renderers.

ergo: it builds faster, produces a smaller image, and updates the base image :sparkles: