[gtk-doc] Update ci README.

[Stefan Sauer <stefkost src gnome org>]

commit 5e3f2cee6a99f720d7acdc7e8803f0cc75c125f7
Author: Stefan Sauer <ensonic users sf net>
Date:   Thu Jul 11 22:37:22 2019 -0700

    Update ci README.

 .gitlab-ci/README.md | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)
---
diff --git a/.gitlab-ci/README.md b/.gitlab-ci/README.md
index 41dfd75..dc6c821 100644
--- a/.gitlab-ci/README.md
+++ b/.gitlab-ci/README.md
@@ -6,18 +6,16 @@ GitLab CI jobs run in a Docker image, defined here. To update that image
 (perhaps to install some more packages):
 
 1. Edit `.gitlab-ci/Dockerfile` with the changes you want
-2. Edit `.gitlab-ci/run-docker.sh` and bump the version in `TAG`
-3. Run `.gitlab-ci/run-docker.sh` to build the new image, and launch a shell
-   inside it
-    * When you're done, exit the shell in the usual way
-4. Run `.gitlab-ci/run-docker.sh --push` to upload the new image to the GNOME
-   GitLab Docker registry
+1. Run `.gitlab-ci/run-docker.sh build --base=debian --base-version=1` to build
+   the new image (bump the version as needed)
+1. Run `.gitlab-ci/run-docker.sh push  --base=debian --base-version=1` to upload
+   the new image to the GNOME GitLab Docker registry
     * If this is the first time you're doing this, you'll need to log into the
       registry
     * If you use 2-factor authentication on your GNOME GitLab account, you'll
       need to [create a personal access token][pat] and use that rather than
       your normal password
-5. Edit `.gitlab-ci.yml` (in the root of this repository) to use your new
+1. Edit `.gitlab-ci.yml` (in the root of this repository) to use your new
    image
 
 [pat]: https://gitlab.gnome.org/profile/personal_access_tokens