Add DISABLE_SPRING environment variable to gitpod forem server task (#15807)

https://www.gitpod.io/docs/environment-variables#terminal-specific-environment-variables

Without this I'm seeing the ENV["APP_DOMAIN"] not set when spring is
loaded (this prevents browsing the site from within gitpod).

.gitpod.yml

@@ -13,6 +13,8 @@ ports:
 
 tasks:
   - name: Forem Server
+    env:
+      DISABLE_SPRING: 1
     before: >
       redis-server &
       gp await-port 5432 &&