title: What is the "gateway" found in `docker inspect`?
body: <p>I have the nginx running in docker(Docker for mac, using docker-compose), here is the "Networks" section of <code>docker inspect</code>. </p>
  
  <pre><code>{
      "Networks": {
          "laradock_backend": {
              "IPAMConfig": null,
              "Links": null,
              "Aliases": [
                  "c189cabxfdf9",
                  "nginx"
              ],
              "NetworkID": "f4f8d8ff07ae90d5758644968d96f2g653fc5188c895f19c2d08de92c46cc075",
              "EndpointID": "f8c6d5a8b061c75c44c2e078a65928a9b45dd91833fc05x7f249c64a180e84a1",
              "Gateway": "172.21.0.1",
              "IPAddress": "172.21.0.4",
              "IPPrefixLen": 16,
              "IPv6Gateway": "",
              "GlobalIPv6Address": "",
              "GlobalIPv6PrefixLen": 0,
              "MacAddress": "02:02:fc:15:10:05",
              "DriverOpts": null
          },
          "laradock_frontend": {
              "IPAMConfig": null,
              "Links": null,
              "Aliases": [
                  "c189cab4ddg1",
                  "nginx"
              ],
              "NetworkID": "7b410b1bd764617a3f6146862307f886681e57aaxf057e4308f1236e1558ffcb",
              "EndpointID": "0caa62bc5bbx600a5b1f260ebg11014e05394671ca347f818bfx819f43f7011e",
              "Gateway": "172.22.0.1",
              "IPAddress": "172.22.0.3",
              "IPPrefixLen": 16,
              "IPv6Gateway": "",
              "GlobalIPv6Address": "",
              "GlobalIPv6PrefixLen": 0,
              "MacAddress": "01:41:af:16:00:03",
              "DriverOpts": null
          }
      }
  }
  </code></pre>
  
  <p>I'm not an expert on computer networking. But I use docker very frequently, I just want to understand a little better about what's happening whenever I connect to the docker container through my host machine (localhost). </p>
  
  <p>Where I found each network has a "Gateway" there.
  I can't find such interfaces on my host machine. Where does the "Gateway" sits? Why we need such a thing? </p>
  
  <p>Any simple charts would be helpful…
  Thanks.</p>