title: auth_backend_http not installed on Rabbit MQ Docker Image 3.7.6
body: <p>I am running <code>rabbitmq:3.7.6</code> <a href="https://hub.docker.com/_/rabbitmq/" rel="nofollow noreferrer">docker image</a> and have the following <code>rabbitmq.conf</code> file:</p>
  
  <pre><code>auth_backends.1         = http
  auth_http.user_path     = http://1.2.3.4/api/rmq/user
  auth_http.vhost_path    = http://1.2.3.4/api/rmq/vhost
  auth_http.resource_path = http://1.2.3.4/api/rmq/resource
  auth_http.topic_path    = http://1.2.3.4/api/rmq/topic
  </code></pre>
  
  <p>When I run the server, it crashes with the error below, leading me to believe <code>auth_backend_http</code> is not installed, even though the docs say it comes with RabbitMQ by default now.</p>
  
  <pre><code>BOOT FAILED
  ===========
  
  Config file generation failed 16:11:41.266 [error] You've tried to set auth_http.user_path, but there is no setting with that name.
  16:11:41.266 [error]   Did you mean one of these?
  16:11:41.279 [error]     ssl_options.depth
  16:11:41.279 [error]     auth_backends.$num
  16:11:41.279 [error]     auth_mechanisms.$name
  16:11:41.279 [error] You've tried to set auth_http.vhost_path, but there is no setting with that name.
  16:11:41.279 [error]   Did you mean one of these?
  16:11:41.292 [error]     auth_backends.$num
  16:11:41.292 [error]     auth_mechanisms.$name
  16:11:41.292 [error]     default_vhost
  16:11:41.292 [error] You've tried to set auth_http.resource_path, but there is no setting with that name.
  16:11:41.292 [error]   Did you mean one of these?
  16:11:41.307 [error]     auth_backends.$num.authn
  16:11:41.307 [error]     auth_backends.$num.authz
  16:11:41.307 [error]     auth_mechanisms.$name
  16:11:41.307 [error] You've tried to set auth_http.topic_path, but there is no setting with that name.
  16:11:41.307 [error]   Did you mean one of these?
  16:11:41.319 [error]     auth_backends.$num
  16:11:41.319 [error]     auth_mechanisms.$name
  16:11:41.319 [error]     net_ticktime
  16:11:41.319 [error] Error generating configuration in phase transform_datatypes
  16:11:41.319 [error] Conf file attempted to set unknown variable: auth_http.topic_path
  16:11:41.319 [error] Conf file attempted to set unknown variable: auth_http.resource_path
  16:11:41.320 [error] Conf file attempted to set unknown variable: auth_http.vhost_path
  16:11:41.320 [error] Conf file attempted to set unknown variable: auth_http.user_path
  </code></pre>
  
  <p><strong>EDIT</strong>: When I <code>RUN ls /plugins</code> I get the following:</p>
  
  <pre><code>README
  ...
  rabbitmq_auth_backend_cache-3.7.6.ez
  rabbitmq_auth_backend_http-3.7.6.ez
  rabbitmq_auth_backend_ldap-3.7.6.ez
  rabbitmq_auth_mechanism_ssl-3.7.6.ez
  </code></pre>