docs: key size estimation steps update

Corrective action for gitlab-com/gl-infra/production#7692 (closed), while executing the steps in gitlab-org/gitlab#370933 (comment 1088260231).

Update the estimation methods to exclude docker, it proved problematic after the rdb dump has grown to 29G, which is bigger than the / volume. It is possible that one sets a non-standard directory to store docker containers, but it's more efficient to build the dump binary and execute it on the host directly.