Unix Domain Socket cannot be specified in DATABASE_URL

**Versions**
- Electric: 1.0.11

**Bug description**
I'm trying to deploy the electric sync-service docker image in a cloud environment (GCP). This environment exposes our postgres server as a unix domain socket. I've tried every single combination of URL's but I simple cannot get electric to pass the URL correctly (see this [stackoverflow](https://stackoverflow.com/a/29356068/3255396).

From what i've read online I would need to use `socket_dir` in Ecto but the docker image does not expose that as an env var: https://github.com/elixir-ecto/postgrex/issues/415

**Expected behavior**
Be able to specify a valid Unix Domain Socket as a database connection