FIX(cache_critical_dns): use DB port sourced from environment

Fixes an assumption that the PostgreSQL port will always be reachable at
the discovered host on the default port when performing the healthcheck.

Instead we should be sourcing this from the same environment variable
that the application will be using to connect to.

Defaults to the standard PG port, 5432.

script/cache_critical_dns

@@ -107,6 +107,7 @@ CRITICAL_HOST_ENV_VARS = %w{
 )
 
 DEFAULT_DB_NAME = "discourse"
+DEFAULT_DB_PORT = 5432
 DEFAULT_REDIS_PORT = 6379
 
 HOST_RESOLVER_CACHE = {}
@@ -267,12 +268,13 @@ ensure
   client.close if client
 end
 
-def postgres_healthcheck(host:, user:, password:, dbname:)
+def postgres_healthcheck(host:, user:, password:, dbname:, port: DEFAULT_DB_PORT)
   client = PG::Connection.new(
     host: host,
     user: user,
     password: password,
     dbname: dbname,
+    port: port,
     connect_timeout: 2,  # minimum
   )
   client.exec(';').none?
@@ -291,12 +293,14 @@ HEALTH_CHECKS = Hash.new(
       host: addr,
       user: ENV["DISCOURSE_DB_USERNAME"] || DEFAULT_DB_NAME,
       dbname: ENV["DISCOURSE_DB_NAME"] || DEFAULT_DB_NAME,
+      port: ENV["DISCOURSE_DB_PORT"] || DEFAULT_DB_PORT,
       password: ENV["DISCOURSE_DB_PASSWORD"])},
   "DISCOURSE_DB_REPLICA_HOST": lambda { |addr|
     postgres_healthcheck(
       host: addr,
       user: ENV["DISCOURSE_DB_USERNAME"] || DEFAULT_DB_NAME,
       dbname: ENV["DISCOURSE_DB_NAME"] || DEFAULT_DB_NAME,
+      port: ENV["DISCOURSE_DB_REPLICA_PORT"] || DEFAULT_DB_PORT,
       password: ENV["DISCOURSE_DB_PASSWORD"])},
   "DISCOURSE_REDIS_HOST": lambda { |addr|
     redis_healthcheck(