feat: Allows sharing of network (#16937)

build(docker): Expose mailhog port.

build(Docker): Improve wording.

build(docker): Mailhog and network changes.

    * Add mailhog container to shared network to enable connectivity
    * Configure project name for docker compose in .env sample file,
        without it the basename of the repo directory is used, which
        makes it unreachable for other services
    * Set mailhog host to mailhog (instead of localhost) if MAILHOG_HOST
      env var is set
    * Expose 1025 to enable local troubleshooting

build(docker): Update README to reflect compose changes.

docker-compose.yml

@@ -0,0 +1,37 @@
+# Docker Compose file for FreeCodeCamp
+#
+# Bootstrap with:
+#   docker-compose run --rm server npm install
+#   docker-compose run --rm server npm run only-once
+#
+# Run with:
+#   docker-compose up
+#
+version: "3"
+services:
+    freecodecamp:
+        image: node:8.9.4
+        depends_on:
+            - db
+            - mailhog
+        environment:
+            - MAILHOG_HOST=mailhog
+            - MONGOHQ_URL=mongodb://db:27017/freecodecamp
+        volumes:
+            - .:/app
+        working_dir: /app
+        command:
+            - npm
+            - run
+            - develop
+        ports:
+            - "3000:3000"
+    db:
+        image: mongo:3.2.6
+        ports:
+            - "27017:27017"
+    mailhog:
+        image: mailhog/mailhog
+        ports:
+            - "1025:1025"
+            - "8025:8025"