docker: add foreman to base image.

Add `foreman` to the base Docker image. This way you can do this:

   docker run --rm -it -v $PWD:/danbooru ghcr.io/danbooru/danbooru foreman start

to start everything needed to run Danbooru in development mode (except
for the Postgres database). This will start everything listed in the
Procfile:

   bin/rails server
   bin/good_job start
   bin/rails danbooru:cron
   bin/webpack-dev-server

config/docker/Dockerfile.danbooru

@@ -27,7 +27,8 @@ RUN \
   asdf plugin add nodejs && \
   asdf install nodejs "$NODE_VERSION" && \
   asdf global nodejs "$NODE_VERSION" && \
-  npm install --unsafe-perm=true -g yarn
+  npm install --unsafe-perm=true -g yarn && \
+  chmod -R go+rx /root/.config # make Yarn config dir readable for non-root
 
 COPY Gemfile Gemfile.lock ./
 RUN \