feat: speed up docs build + extend .dockerignore

Description

Pipeline build was slightly slow - ~7min

Now pipeline build runs ~3.5min

GitLab CI now stores node_modules/.cache and use prev docker images as cache too

Pipeline will be slower if cache will be invalidated - by changing branch, deps, src or Dockerfile

Additionally, .dockerignore was extended to exclude more items that are not needed in Docker images.

Docker version in pipeline and in build step updated from 18.09.7 to 20.10.12 - just a routine update

Related issue

Examples

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes and they have been reviewed OR there are no API changes