Link docker instructions from readme

So glad to see that docker is documented really well! Wondering if it would make more sense to be more DRY in the docs, and simply link to the instructions in slackarachive-docker rather than reproducing them in the slackarchive repo README :)

Start by reading the slackarchive repository README and locate the reproduced Docker instructions. Compare them with the instructions in slackarchive-docker, then replace the duplicated README content with a link if the documentation is still available there. Done means the README points readers to the Docker instructions without maintaining a duplicate.

Written by the indexing model from the issue text.