A Docker container will be launched that will update the cobalt.dev source files in the Cobalt repository specified by $COBALT_SRC, which must be set, and it will locally host the updated cobalt.dev on port 4000.
A separate docker-compose.yml was created due to the root docker-compose.yml being unable to locate the Gemfile in the Docker build context.
The Gemfile was copied from third_party/repo-publishing-toolkit/Gemfile because we need to run bundle install with elevated permissions but if we let the preview-site.sh, which normally installs gems, run with elevated permissions the resulting files added or modified would not be accessible to the user who ran the Docker container.
docker-compose build --build-arg UID=$(id -u) --build-arg GID=$(id -g) docsite docker-compose up docsite