81番ポートでdockerコンテナのapacheにアクセスするとphpが実行されずファイルダウンロードになる

docker初心者です。
81番ポートでapacheコンテナを動かそうとしているのですが、http://xxx.xxx.xxx:81でアクセスすると、以下の内容のファイルがダウンロードされてしまいます。
（Laravelのオートローダーのようです。）

<?php

/**
 * Laravel - A PHP Framework For Web Artisans
 *
 * @package  Laravel
 * @author   Taylor Otwell <taylor@laravel.com>
 */

define('LARAVEL_START', microtime(true));

/*
|--------------------------------------------------------------------------
| Register The Auto Loader
|--------------------------------------------------------------------------
|
| Composer provides a convenient, automatically generated class loader for
| our application. We just need to utilize it! We'll simply require it
| into the script here so that we don't have to worry about manual
| loading any of our classes later on. It feels great to relax.
|
*/

require __DIR__.'/../vendor/autoload.php';

/*
|--------------------------------------------------------------------------
| Turn On The Lights
|--------------------------------------------------------------------------
|
| We need to illuminate PHP development, so let us turn on the lights.
| This bootstraps the framework and gets it ready for use, then it
| will load up this application so that we can run it and send
| the responses back to the browser and delight our users.
|
*/

$app = require_once __DIR__.'/../bootstrap/app.php';

/*
|--------------------------------------------------------------------------
| Run The Application
|--------------------------------------------------------------------------
|
| Once we have the application, we can handle the incoming request
| through the kernel, and send the associated response back to
| the client's browser allowing them to enjoy the creative
| and wonderful application we have prepared for them.
|
*/

$kernel = $app->make(Illuminate\Contracts\Http\Kernel::class);

$response = $kernel->handle(
    $request = Illuminate\Http\Request::capture()
);

$response->send();

$kernel->terminate($request, $response);

docker ps の結果は以下です。

[root@os3-374-20202 httpd]# docker ps
CONTAINER ID        IMAGE                 COMMAND                  CREATED             STATUS              PORTS                                      NAMES
bb54e119662c        75d6aabd27d6          "/sbin/init"             29 minutes ago      Up 29 minutes       80/tcp, 0.0.0.0:81->81/tcp                 ga_k8s-test1
b5b9a182ca27        laradock_beanstalkd   "/usr/bin/beanstalkd"    3 hours ago         Up 3 hours          0.0.0.0:11300->11300/tcp                   laradock_beanstalkd_1
54de35884935        laradock_nginx        "/bin/bash /opt/star…"   3 hours ago         Up 3 hours          0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp   laradock_nginx_1
fce505b944c5        laradock_php-fpm      "docker-php-entrypoi…"   3 hours ago         Up 3 hours          9000/tcp                                   laradock_php-fpm_1
cc2c20deff49        laradock_workspace    "/sbin/my_init"          3 hours ago         Up 3 hours          0.0.0.0:2222->22/tcp                       laradock_workspace_1
5bb8870c05d4        laradock_mysql        "docker-entrypoint.s…"   3 hours ago         Up 3 hours          0.0.0.0:3306->3306/tcp, 33060/tcp          laradock_mysql_1
23968ee56bc4        docker:dind           "dockerd-entrypoint.…"   3 hours ago         Up 3 hours          2375/tcp                                   laradock_docker-in-docker_1
ef4e1aa9c143        laradock_redis        "docker-entrypoint.s…"   3 hours ago         Up 3 hours          0.0.0.0:6379->6379/tcp                     laradock_redis_1

※一番上の ga_k8s-test1 が問題のコンテナです。

ga-k8s-test1 の中の /etc/httpd/conf.d/php.conf の記述が以下です。

[root@bb54e119662c conf.d]# cat php.conf
#
# The following lines prevent .user.ini files from being viewed by Web clients.
#
<Files ".user.ini">
    <IfModule mod_authz_core.c>
        Require all denied
    </IfModule>
    <IfModule !mod_authz_core.c>
        Order allow,deny
        Deny from all
        Satisfy All
    </IfModule>
</Files>

#
# Allow php to handle Multiviews
#
AddType text/html .php

#
# Add index.php to the list of files that will be served as directory
# indexes.
#
DirectoryIndex index.php

# mod_php options
<IfModule  mod_php7.c>
    #
    # Cause the PHP interpreter to handle files with a .php extension.
    #
    <FilesMatch .(php|phar)$>
        SetHandler application/x-httpd-php
    </FilesMatch>

    #
    # Uncomment the following lines to allow PHP to pretty-print .phps
    # files as PHP source code:
    #
    #<FilesMatch .phps$>
    #    SetHandler application/x-httpd-php-source
    #</FilesMatch>

    #
    # Apache specific PHP configuration options
    # those can be override in each configured vhost
    #
    php_value session.save_handler "files"
    php_value session.save_path    "/var/lib/php/session"
    php_value soap.wsdl_cache_dir  "/var/lib/php/wsdlcache"

    #php_value opcache.file_cache   "/var/lib/php/opcache"
</IfModule>

ホスト側でもコンテナ側でも、curl http://localhost:81 を実行するとちゃんとhtmlファイルが返ってきます。
どうしてブラウザから http://xxx.xxx.xxx:81 (xxx.xxx.xxxはドメイン名) を叩くと、ファイルのダウンロードになってしまうのでしょうか。