[glib/wjt/g-file-info-filesystem-readonly-fails] ci: document how to update the Docker image

[Will Thompson <wjt src gnome org>]

commit 1520afb451550b05a1fe0a102fd5d99817069ccc
Author: Will Thompson <will willthompson co uk>
Date:   Tue Nov 13 10:32:35 2018 +0000

    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     | 23 +++++++++++++++++++++++
 .gitlab-ci/run-docker.sh |  1 +
 2 files changed, 24 insertions(+)
---
diff --git a/.gitlab-ci/README.md b/.gitlab-ci/README.md
new file mode 100644
index 000000000..41dfd7559
--- /dev/null
+++ b/.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
diff --git a/.gitlab-ci/run-docker.sh b/.gitlab-ci/run-docker.sh
index f9a8c7513..a0caec54a 100755
--- a/.gitlab-ci/run-docker.sh
+++ b/.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" .