docs(README): Move Environment variables above build args

docker build --build-arg BASE=docker.io/sharelatex/sharelatex:2.6.1 -t docker-overleaf-ldap .

```

## Environment variables

Two environment variables are used at runtime to configure the bind user:

- `LDAP_BIND_USER`: Bind-DN, i.e., DN of the bind user.

- `LDAP_BIND_PW`: Password of the bind user.

## Build arguments

The following arguments can be passed via `--build-args`.

| `BASE`            | `docker.io/sharelatex/sharelatex`                                     | Can be set to any Overleaf image tag. See [here](https://hub.docker.com/r/sharelatex/sharelatex/tags?page=1&ordering=last_updated) for a list of tags. |

| `LDAP_PLUGIN_URL` | `https://codeload.github.com/smhaller/ldap-overleaf-sl/tar.gz/master` | URL to download ldap-overleaf-sl from.                                                                                                                 |

## Environment variables

Two environment variables are used to configure the bind user:

- `LDAP_BIND_USER`: Bind-DN, i.e., DN of the bind user.

- `LDAP_BIND_PW`: Password of the bind user.

## GitLab CI

The `environment` file is used to specify some environment variables for the GitLab CI: