Add CMD and refactor ENTRYPOINT in the Docker generator (rails#46778)

* Add CMD and refactor ENTRYPOINT

The CMD will run by default when you run the container without supplying
any arguments but you can choose to overwrite this at runtime by
doing: docker container run -it myapp rails c

The ENTRYPOINT will only create or migrate the database when the default
CMD is invoked. If you choose to run a different command such as the
rails console or a background worker process the db:prepare command
will not run.

* Remove extra space from ENTRYPOINT comment

Author: nickjj

railties/lib/rails/generators/rails/app/templates/Dockerfile.tt

@@ -44,7 +44,9 @@ RUN bundle exec bootsnap precompile --gemfile app/ lib/
 # Precompiling assets for production without requiring secret RAILS_MASTER_KEY
 RUN SECRET_KEY_BASE_DUMMY=1 bundle exec rails assets:precompile
 
-# Entrypoint prepares database and starts app on 0.0.0.0:3000 by default,
-# but can also take a rails command, like "console" or "runner" to start instead.
+# Entrypoint prepares the database.
 ENTRYPOINT ["/rails/bin/docker-entrypoint"]
+
+# Start the server by default, this can be overwritten at runtime.
 EXPOSE 3000
+CMD ["./bin/rails", "server"]

railties/lib/rails/generators/rails/app/templates/docker-entrypoint.tt

@@ -1,12 +1,8 @@
 #!/bin/sh
 
-if [ $# -eq 0 ]; then
-  # Create new or migrate existing database
+# If running the rails server then create or migrate existing database
+if [ "${*}" == "./bin/rails server" ]; then
   ./bin/rails db:prepare
-
-  # Start the server by default
-  exec bin/rails server
-else
-  # Allow other commands, like console or runner, to be called
-  exec bin/rails "$@"
 fi
+
+exec "${@}"