Rely on depends_on for docker tox env

tox.ini

@@ -42,18 +42,16 @@ commands =
 commands_post =
 
 [testenv:docker]
+commands_pre =
 whitelist_externals=
     docker-compose
-commands_pre =
-    docker-compose up --build --detach mysql redis rabbitmq
 commands =
     docker-compose run -e TREEHERDER_DEBUG=False backend bash -c "pytest --cov --cov-report=xml tests/ --runslow -p no:unraisableexception"
 
 [testenv:docker-postgres]
+commands_pre =
 whitelist_externals=
     docker-compose
-commands_pre =
-    docker-compose up --build --detach postgres redis rabbitmq
 commands =
     docker-compose run -e TREEHERDER_DEBUG=False -e DATABASE_URL=psql://postgres:mozilla1234@postgres:5432/treeherder backend bash -c "pytest --cov --cov-report=xml tests/ --runslow -p no:unraisableexception"