Docs about `mlem build docker`

[aguschin]

1. I can't see mlem build docker here https://mlem.ai/doc/user-guide/building/docker/, nor it mentions that it's covered in the page above https://mlem.ai/doc/user-guide/building/
2. This page goes right into using declarations, which is something we shouldn't do to avoid confusing novices. We need to show just mlem build docker, or redirect to GS.