The Keystatic's environment variables must be defined during the build.
@prastoin and I identified that the other environment variables aren't
defined during the build. We decided to add fake environment variables
directly in the Dockerfile to make the build pass. Later, the docker
image should be executed with the real environment variables that'll
make Keystatic work properly.
Having 2 different dev setups caused confusion, let's remove the Docker
local setup and recommend people install yarn locally.
Also simplified some docs by merging pages together, the recommend
self-hosting option is now the docker-compose / to adapt the
docker-compose.
