[ci] Streamlined Docker image selection logic.

.gitlab-ci.yml

scripts/ci/jobs/build-docs.yml

scripts/ci/jobs/build-gui-linux.yml

scripts/ci/jobs/build-linux.yml

scripts/ci/jobs/check-header.yml

scripts/ci/jobs/ci_images.yml

scripts/ci/jobs/clang-sanitizer.yml

scripts/ci/jobs/clang-tidy.yml

scripts/ci/jobs/code-quality.yml

scripts/ci/jobs/meta.yml

scripts/ci/jobs/pre-commit.yml

scripts/ci/jobs/web.yml