gitlab-ci: drop "tag"/"default_tag" from ci templates

The tag we actually use already contains a hash of the input files and
is generated (by `ci-fairy generate-templates`). There is no need for having
this fixed prefix. As also seens by having a date there, which is maintained
badly and meaningless.

Drop it.

.gitlab-ci.yml

@@ -57,11 +57,11 @@ variables:
   #
   # This is done by running `ci-fairy generate-template` and possibly bumping
   # ".default_tag".
-  ALPINE_TAG:  '2023-04-11.0-63b57b80e21f'
-  CENTOS_TAG:  '2023-04-11.0-b6ce4d55572b'
-  DEBIAN_TAG:  '2023-04-11.0-de33b560b02a'
-  FEDORA_TAG:  '2023-04-11.0-b6ce4d55572b'
-  UBUNTU_TAG:  '2023-04-11.0-de33b560b02a'
+  ALPINE_TAG:  'tag-0a615b61395f'
+  CENTOS_TAG:  'tag-87fa88557da7'
+  DEBIAN_TAG:  'tag-2edf40356382'
+  FEDORA_TAG:  'tag-87fa88557da7'
+  UBUNTU_TAG:  'tag-2edf40356382'
 
   ALPINE_EXEC: 'bash .gitlab-ci/alpine-install.sh'
   CENTOS_EXEC: 'bash .gitlab-ci/fedora-install.sh'

.gitlab-ci/ci.template

@@ -17,10 +17,10 @@
 
 .templates_sha: &template_sha ffe4d1b10aab7534489f0c4bbc4c5899df17d3f2 # see https://docs.gitlab.com/ee/ci/yaml/#includefile
 
-{# Group distros by their common (name,base_type,tag) tuples.#}
+{# Group distros by their common (name,base_type) tuples.#}
 {% set distro_groups = [] %}
 {% for distro in distributions %}
-  {% set g = {'name':distro.name,'base_type':distro.base_type,'tag':distro.tag} %}
+  {% set g = {'name':distro.name,'base_type':distro.base_type} %}
   {% if g not in distro_groups %}
   {%   do distro_groups.append(g) %}
   {% endif %}
@@ -62,7 +62,7 @@ variables:
   # This is done by running `ci-fairy generate-template` and possibly bumping
   # ".default_tag".
 {% for distro_group in distro_groups|sort(attribute='name') %}
-  {{"%-13s"| format(distro_group.name.upper() + '_TAG:')}}'{{distro_group.tag}}-{{
+  {{"%-13s"| format(distro_group.name.upper() + '_TAG:')}}'tag-{{
     (ci_fairy.hashfiles('./.gitlab-ci/config.yml',
                         './.gitlab-ci/ci.template',
                         './.gitlab-ci/' + distro_group.base_type + '-install.sh',

.gitlab-ci/config.yml

@@ -7,29 +7,21 @@
 # https://gitlab.freedesktop.org/freedesktop/ci-templates
 #
 
-# We're happy to rebuild all containers when one changes.
-.default_tag: &default_tag '2023-04-11.0'
-
-
 # The list of all distributions we want to create job for.
-# The template generates manual jobs for all these.
 distributions:
   - name: fedora
-    tag: *default_tag
     base_type: fedora
-    # Tier1 is used to build the pages and check-{tree,patch}
+    # The first tier:1 in the list is used to build the pages and check-{tree,patch}
     tier: 1
     versions:
       - '37'
   - name: fedora
-    tag: *default_tag
     base_type: fedora
     tier: 2
     versions:
       - '36'
       - '38'
   - name: fedora
-    tag: *default_tag
     base_type: fedora
     tier: 3
     versions:
@@ -40,7 +32,6 @@ distributions:
       - '34'
       - '35'
   - name: ubuntu
-    tag: *default_tag
     base_type: debian
     tier: 2
     versions:
@@ -50,7 +41,6 @@ distributions:
       - 'devel'
       - 'rolling'
   - name: debian
-    tag: *default_tag
     base_type: debian
     tier: 2
     versions:
@@ -58,14 +48,12 @@ distributions:
       - '11'
       - 'sid'
   - name: debian
-    tag: *default_tag
     base_type: debian
     tier: 3
     versions:
       - '9'
       - 'testing'
   - name: centos
-    tag: *default_tag
     base_type: fedora
     tier: 2
     versions:
@@ -74,7 +62,6 @@ distributions:
       - '8.1.1911'
       - '8.3.2011'
   - name: centos
-    tag: *default_tag
     base_type: fedora
     tier: 3
     versions:
@@ -83,7 +70,6 @@ distributions:
       - '7.8.2003'
       - '8.2.2004'
   - name: alpine
-    tag: *default_tag
     base_type: alpine
     tier: 2
     versions: