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-shared.yml

@@ -0,0 +1,20 @@
+# Docker Compose over ride file for FreeCodeCamp
+#
+# Run with:
+#   docker-compose -f docker-compose.yml -f docker-compose-shared.yml up
+#
+version: "3"
+services:
+    db:
+        networks:
+            - shared
+    mailhog:
+        networks:
+            - shared
+    freecodecamp:
+        networks:
+            - shared
+
+networks: # Used by by other projects like open-api
+    shared:
+        driver: bridge