Use the default branch for Docker image CI builds

This allows to use 'main' as default branch without
modifying the CI template.

It also uses CI rules instead of only/except.

changelogs/unreleased/ci-templates-docker-main.yml

+---
+title: Unify the Docker Image build CI template and use the default branch instead
+  of hardcoded 'master'
+merge_request: 51931
+author: dnsmichi
+type: changed

lib/gitlab/ci/templates/Docker.gitlab-ci.yml

-docker-build-master:
-  # Official docker image.
-  image: docker:latest
-  stage: build
-  services:
-    - docker:dind
-  before_script:
-    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
-  script:
-    - docker build --pull -t "$CI_REGISTRY_IMAGE" .
-    - docker push "$CI_REGISTRY_IMAGE"
-  only:
-    - master
-
+# Build a Docker image with CI/CD and push to the GitLab registry.
+# Docker-in-Docker documentation: https://docs.gitlab.com/ee/ci/docker/using_docker_build.html
+#
+# This template uses one generic job with conditional builds
+# for the default branch and all other (MR) branches.
 docker-build:
-  # Official docker image.
+  # Use the official docker image.
   image: docker:latest
   stage: build
   services:
     - docker:dind
   before_script:
     - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  # Default branch leaves tag empty (= latest tag)
+  # All other branches are tagged with the escaped branch name (commit ref slug)
   script:
-    - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG" .
-    - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG"
-  except:
-    - master
+    - |
+      if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
+        tag=""
+        echo "Running on default branch '$CI_DEFAULT_BRANCH': tag = 'latest'"
+      else
+        tag=":$CI_COMMIT_REF_SLUG"
+        echo "Running on branch '$CI_COMMIT_BRANCH': tag = $tag"
+      fi
+    - docker build --pull -t "$CI_REGISTRY_IMAGE${tag}" .
+    - docker push "$CI_REGISTRY_IMAGE${tag}"
+  # Run this job in a branch where a Dockerfile exists
+  rules:
+    - if: $CI_COMMIT_BRANCH
+      exists:
+        - Dockerfile