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

Procfile

@@ -7,8 +7,19 @@
 # http://blog.daviddollar.org/2011/05/06/introducing-foreman.html
 # https://github.com/ddollar/foreman
 
-web: bin/rails server
+# The main webserver. See config/puma.rb and https://github.com/puma/puma.
+# unset PORT to workaround a Puma+Foreman issue (https://github.com/puma/puma/issues/1771)
+web: unset PORT && bin/rails server
+
+# The background job worker. See app/jobs/ and https://github.com/bensheldon/good_job.
 worker: bin/good_job start
+
+# The cron job worker. See config/initializers/clockwork.rb and https://github.com/Rykian/clockwork.
 clock: bin/rails danbooru:cron
+
+# The Javascript bundler. Rebuilds Javascript/CSS files when they change. See
+# config/webpacker.yml and https://webpack.js.org/configuration/dev-server.
 webpack-dev-server: bin/webpack-dev-server
-# db: docker run --rm -it --name danbooru-postgres --shm-size=8g -p 5432:5432 -e POSTGRES_USER=danbooru - e POSTRES_HOST_AUTH_METHOD=trust -v danbooru-postgres:/var/lib/postgresql/data ghcr.io/danbooru/postgres:14.0
+
+# The postgres database. It can be run in the Procfile, but it's better to run it manually.
+# db: docker run --rm -it --name danbooru-postgres --shm-size=8g -p 5432:5432 -e POSTGRES_USER=danbooru - e POSTRES_HOST_AUTH_METHOD=trust -v danbooru-postgres:/var/lib/postgresql/data ghcr.io/danbooru/postgres:14.1