convert contract tests to using docker bridge network

refs DE-715

Test Plan
1. Contract Tests passes

[canvas-builds-refspec=755cccfd95de9aafca779c0a1f5697fc9eb40b6f]

Change-Id: I077ae015ca7c50cf4d8774958438a948649c2fbf
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/267131
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Kyle Rosenbaum <krosenbaum@instructure.com>
QA-Review: Aaron Ogata <aogata@instructure.com>
Product-Review: Aaron Ogata <aogata@instructure.com>

Jenkinsfile.contract-tests

@@ -36,7 +36,6 @@ pipeline {
     PACT_BROKER = credentials('PACT_BROKER')
     PACT_BROKER_USERNAME = "${env.PACT_BROKER_USR}"
     PACT_BROKER_PASSWORD = "${env.PACT_BROKER_PSW}"
-    RAILS_DB_HOST = 'localhost'
     RSPEC_PROCESSES = '7'
   }
 

build/new-jenkins/library/vars/contractTestsStage.groovy

@@ -32,7 +32,6 @@ def nodeRequirementsTemplate() {
       PACT_BROKER_PROTOCOL: 'https',
       POSTGRES_PASSWORD: env.POSTGRES_PASSWORD,
       RAILS_ENV: 'test',
-      RAILS_DB_HOST: env.RAILS_DB_HOST,
       RANDOMIZE_SEQUENCES: 1,
     ]
   ]
@@ -55,9 +54,7 @@ def nodeRequirementsTemplate() {
           PGDATA: '/data',
           POSTGRES_PASSWORD: env.POSTGRES_PASSWORD,
         ],
-        ports: [
-          postgres: 5432,
-        ]
+        ports: [5432],
       ]
     ],
   ]
@@ -82,7 +79,7 @@ def queueTestStage(stageName) {
   { opts, stages ->
     def baseEnvVars = [
       "DATABASE_NAME=${opts.databaseName}",
-      "DATABASE_URL=postgres://postgres:${env.POSTGRES_PASSWORD}@${env.RAILS_DB_HOST}:5432/${opts.databaseName}",
+      "DATABASE_URL=postgres://postgres:${env.POSTGRES_PASSWORD}@postgres:5432/${opts.databaseName}",
       "PACT_API_CONSUMER=${opts.containsKey('consumerName') ? opts.consumerName : ''}",
     ]