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/build-base-image.sh

@@ -139,6 +139,10 @@ install_postgresql_client() {
   apt_install postgresql-client-${POSTGRESQL_CLIENT_VERSION}
 }
 
+install_foreman() {
+  gem install foreman
+}
+
 install_busybox() {
   busybox --install -s
 }
@@ -172,5 +176,6 @@ install_vips
 install_ruby
 install_openresty
 install_postgresql_client
+install_foreman
 cleanup
 install_busybox # after cleanup so we can install some utils removed by cleanup