DEV: Add docker:test:setup Rake task (#23430)

## What is the context here?

The `docker.rake` Rakefile contains Rake tasks that are meant to be run
in the `discourse/discourse_test:release` Docker image. For example, we
have the `docker:test` Rake task that makes it easier to run the test
suite for a particular Discourse commit.

Why are we introducing a `docker:test:setup` Rake task?

While we have the `docker:test` Rake task, it is very limited in the
test commands that can be executed. It is very useful for automated
testing but not very useful for running tests in the development
environment. Therefore, we are introducing a `docker:test:setup` rake
task that can be used to set up the test environment for running tests.

The envisioned example usage is something like this:

```
docker run -d --name=discourse_test --entrypoint=/sbin/boot discourse/discourse_test:release
docker exec -u discourse:discourse discourse_test ruby script/docker_test.rb --no-tests
docker exec -u discourse:discourse discourse_test bundle exec rake docker:test:setup
docker exec -u discourse:discourse discourse_test bundle exec rspec <path to file>
```

script/start_test_db.rb

@@ -24,6 +24,8 @@ while a = ARGV.pop
 end
 
 if should_setup
+  run "rm -rf #{DATA}"
+  run "mkdir -p #{DATA}"
   run "#{BIN}/initdb -D #{DATA}"
 
   run "echo fsync = off >> #{DATA}/postgresql.conf"
@@ -32,7 +34,7 @@ if should_setup
 end
 
 if should_exec
-  exec "#{BIN}/postmaster -D #{DATA}"
+  exec "#{BIN}/postgres -D #{DATA}"
 elsif should_run
   run "#{BIN}/pg_ctl -D #{DATA} start"
 end