🔧(ci) allow env override in compose for e2e tests

Add an ENV_OVERRIDE variable so docker-compose can load an
extra env file (e.g. common.e2e) on top of common.local.
This lets the e2e runner enable WOPI_CLIENTS=collabora
without touching the default dev config.

compose.yaml

@@ -123,6 +123,7 @@ services:
     env_file:
       - env.d/development/common
       - env.d/development/common.local
+      - env.d/development/common.${ENV_OVERRIDE:-local}
       - env.d/development/postgresql
       - env.d/development/postgresql.local
     volumes:

env.d/development/common.e2e

@@ -1 +1,2 @@
 FRONTEND_RELEASE_NOTE_ENABLED=False
+WOPI_CLIENTS="collabora"