title: Cannot load and use synonym.txt file on ElasticSearch
body: <p>I am using the following docker-compose.yml to create an ElasticSearch cluster:</p>
  
  <pre><code>version: '3.3'
  services:
    elasticsearch1:
      image: docker.elastic.co/elasticsearch/elasticsearch:7.6.2
      container_name: elasticsearch1
      environment:
        ...
      ulimits:
        memlock:
          soft: -1
          hard: -1
      volumes:
        - data01:/usr/share/elasticsearch/data
        - ${PWD}/data/thesaurus/output:/usr/share/elasticsearch/config/extra
      ports:
        - 9200:9200
      networks:
        - elastic
    elasticsearch2:
      ...
  volumes:
    data01:
      driver: local
    data02:
      driver: local
  networks:
    elastic:
      driver: bridge
  </code></pre>
  
  <p>My index contains:</p>
  
  <pre><code>  "settings": {
      "number_of_shards": 2,
      "number_of_replicas": 1,
      "index" : {
        "analysis" : {
            "filter" : {
                "synonym" : {
                    "type" : "synonym", 
                    "synonyms_path" : "extra/synonym.txt"
                }
            },
            "analyzer" : {
                "synonym" : {
                    "tokenizer" : "standard",
                    "filter" : ["lowercase", "synonym"] 
                }
            }
        }
      }
  </code></pre>
  
  <p>When I try to <code>PUT</code> my data, I get: </p>
  
  <pre class="lang-py prettyprint-override"><code>{'error': {'root_cause': [{'type': 'illegal_argument_exception', 'reason': 'failed to build synonyms'}], 'type': 'illegal_argument_exception', 'reason': 'failed to build synonyms', 'caused_by': {'type': 'i_o_exception', 'reason': 'Is a directory'}}, 'status': 400}
  </code></pre>
  
  <p>Interestingly, when I run: <code>docker exec elasticsearch1 cat config/extra/synonym.txt</code>, I get the error: <code>cat: config/extra/synonym.txt: Is a directory</code></p>
  
  <p>How do I load and use the synonym.txt file?</p>