Backup from external PG doesn't work for postgres 10.4

Summary

Backup from external PG doesn't work for postgres 10.4

This is because the pg_dump shipped in the task-runner is too old.

We should come up with a workaround, and at least document the solution.

At the moment if someone wanted to work around it, I think you would need to build a new docker image, based on the one being used by the task runner, that installs the proper pg_dump. Then set the task runner to use that image.

See conversations here: https://gitlab.slack.com/team/U8QP8KCA2 (GitLab internal only)