fix(docker): app and sidekiq containers ignore signals

Typically, when attempting to stop a container via `docker stop` or
`podman stop`, the container engine will send a stop signal (SIGTERM by
default) to the container's main process. There are two common ways this
can go wrong:

1. The main process is run as PID 1 and doesn't register a signal
   handler for the stop signal and is consequently ignored
2. The main process is a shell script running a foreground process with
   no `trap`s and is consequently ignored by the *shell*

In either case, because the graceful stop signal is ignored, the
container engine will then send a `SIGKILL` to the container process
after a default timeout of 10 seconds. This is why some containers can
be observed to "hang" when being stopped when they have no other reason
to do so. Unlike `SIGTERM` or `SIGINT`, `SIGKILL` is an immediate,
ungraceful stop that doesn't give the process time to clean up.

There is a fair bit of nuance in how `sh` and `bash` handle signals in
different circumstances. The behavior relevant to the second case above
and Dawarich's entrypoints in particular is that the shell ignores
signals like `SIGTERM` and `SIGINT` when waiting on a foreground job; in
this case, that would be: `bundle exec ${@}`. The reason that `SIGINT`
is not ignored after pressing `Ctrl+C` while running the docker compose
stack is because in that case the shell is **interactive** and the shell
*does* respond to `SIGINT` then (c.f. the aforementioned nuance).

Thankfully, the fix is simple: `exec` the main process, which causes the
server process to *replace* the shell process and directly receive any
signals sent. Additionally, the stop signal for the app process should
be set to `SIGINT`, as that is the expected signal for graceful
shutdown. Sidekiq is fine with either `SIGTERM` or `SIGINT`, which is
convenient.

docker/Dockerfile

@@ -80,4 +80,6 @@ RUN chmod +x /usr/local/bin/sidekiq-entrypoint.sh
 
 EXPOSE $RAILS_PORT
 
+STOPSIGNAL SIGINT
+
 ENTRYPOINT [ "bundle", "exec" ]

docker/sidekiq-entrypoint.sh

@@ -33,4 +33,4 @@ done
 echo "✅ PostgreSQL is ready!"
 
 # run sidekiq
-bundle exec sidekiq
+exec bundle exec sidekiq

docker/web-entrypoint.sh

@@ -83,4 +83,4 @@ echo "Running seeds..."
 bundle exec rails db:seed
 
 # run passed commands
-bundle exec ${@}
+exec bundle exec ${@}