Build containers with GitLab CI without root nor daemons by using buildkit

.gitlab-ci.yml

build-container:
  stage: build
  image:
    name: moby/buildkit:rootless
    entrypoint: [ "sh", "-c" ]
  variables:
    BUILDKITD_FLAGS: --oci-worker-no-process-sandbox
  before_script:
  - |
    mkdir ~/.docker
    auth=$(echo -n "$CI_REGISTRY_USER:$CI_REGISTRY_PASSWORD" | base64)
    echo "{\"auths\": {\"$CI_REGISTRY\": {\"auth\": \"$auth\"}}}" > ~/.docker/config.json
  script:
  - |
    buildctl-daemonless.sh build \
        --frontend=dockerfile.v0 \
        --local context=. \
        --local dockerfile=. \
        --import-cache type=registry,ref=$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG \
        --export-cache type=inline \
        --output type=image,name=$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG,push=true

See also: https://gitlab.com/txlab/ci-templates