ci: Move GST_UPSTREAM_BRANCH into the images_template.yml

Used for determining which version of gstreamer we will build
in the docker images. Move it along ther other variables so
it will be easier to update.

Also change ci/install-gst.sh to avoid hardoding the version
and instead use the variable like the windows build does.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1433>

.gitlab-ci.yml

@@ -514,7 +514,6 @@ pages:
     # We also don't need a CONTEXT_DIR var as its also
     # hardcoded to be windows-docker/
     DOCKERFILE: 'ci/windows-docker/Dockerfile'
-    GST_UPSTREAM_BRANCH: 'main'
   tags:
     - 'windows'
     - 'shell'

ci/images_template.yml

@@ -2,3 +2,6 @@ variables:
   GST_RS_IMG_TAG: "2024-04-10.0"
   GST_RS_STABLE: "1.77.2"
   GST_RS_MSRV: "1.70.0"
+  # The branch we use to build GStreamer from in the docker images
+  # Ex. main, 1.24, my-test-branch
+  GST_UPSTREAM_BRANCH: 'main'

ci/install-gst.sh

@@ -2,6 +2,8 @@
 
 set -e
 
+DEFAULT_BRANCH="$GST_UPSTREAM_BRANCH"
+
 pip3 install meson==1.1.1 --break-system-packages
 
 # gstreamer-rs already has a 'gstreamer' directory so don't clone there
@@ -9,7 +11,7 @@ pushd .
 cd ..
 git clone https://gitlab.freedesktop.org/gstreamer/gstreamer.git \
     --depth 1 \
-    --branch 1.24
+    --branch "$DEFAULT_BRANCH"
 
 cd gstreamer