Make gateway host configurable via SERVER_HOST environment variable

.run/npm-dev.run.xml

@@ -9,6 +9,7 @@
     <envs>
       <env name="HCAPTCHA_SECRET" value="10000000-ffff-ffff-ffff-000000000001" />
       <env name="USE_LOCAL_GATEWAY" value="true" />
+      <env name="SERVER_HOST" value="localhost" />
     </envs>
     <method v="2" />
   </configuration>

backend/gateway/src/main/resources/application.yml

@@ -11,7 +11,7 @@ spring:
     gateway:
       routes:
         - id: server
-          uri: http://localhost:8081
+          uri: http://${SERVER_HOST:localhost}:8081
           predicates:
             - Path=/api/**
           filters: