Docker install instructions

[manuelmaurer]

The readme states there is an installation via docker available.
While this is technically true (I don't think the github repo is very intuitive), I would recommend adding a docker section to the installation tab here: https://www.speakeasy.com/docs/sdks/create-client-sdks#install-the-speakeasy-cli with something like
docker pull ghcr.io/speakeasy-api/speakeasy