ci: document how to update the Docker image

By making run-docker.sh to `cd` to the correct directory, we don't need to
document that step.

.gitlab-ci/README.md

@@ -0,0 +1,23 @@
+# CI support stuff
+
+## Docker image
+
+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
+    * 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
+   image
+
+[pat]: https://gitlab.gnome.org/profile/personal_access_tokens

.gitlab-ci/run-docker.sh

@@ -4,6 +4,7 @@ set -e
 
 TAG="registry.gitlab.gnome.org/gnome/glib/master:v10"
 
+cd "$(dirname "$0")"
 docker build --build-arg HOST_USER_ID="$UID" --tag "${TAG}" \
     --file "Dockerfile" .