Stick with Alpine Linux 3.17 in the GitLab CI images

Alpine Linux just released new docker image 3.18, updating latest from
3.17 [1], which causes the GNOME GitLab CI jobs to fail like this:

    $ apk add gnome-common yelp-tools automake autoconf glib-dev libtool g++ parted-dev gtkmm3-dev itstool make git polkit-dev
    ERROR: unable to select packages:
      gnome-common (no such package):
        required by: world[gnome-common]

Based on this blog [2] I printed the apk repository configuration file
in Alpine Linux 3.17 and 3.18 docker images in GitLab CI jobs and found
this contents:

Alpine Linux 3.17
    $ cat /etc/apk/repositories
    https://dl-cdn.alpinelinux.org/alpine/v3.17/main
    https://dl-cdn.alpinelinux.org/alpine/v3.17/community

Alpine Linux 3.18
    $ cat /etc/apk/repositories
    https://dl-cdn.alpinelinux.org/alpine/v3.18/main
    https://dl-cdn.alpinelinux.org/alpine/v3.18/community

So what I conclude is that the packages GParted needs aren't yet
available or being distributed for Alpine Linux 3.18.  Therefore fix
this by explicitly sticking with Alpine Linux 3.17.

[1] Alpine docker images
    https://hub.docker.com/_/alpine/
    * 3.18.0, 3.18, 3, latest
    571516c7ce/x86_64/Dockerfile
    Committed 2023-May-09

[2] ERROR: unable to select packages error on Alpine Linux
    https://www.hasanaltin.com/error-unable-to-select-packages-error-on-alpine-linux/

.gitlab-ci.yml

@@ -4,7 +4,7 @@ stages:
 
 .alpine_image_template: &alpine_image_definition
   # Use official Alpine image https://hub.docker.com/_/alpine/.
-  image: alpine:latest
+  image: alpine:3.17
   before_script:
     - cat /proc/version
     - cat /etc/os-release