Simple GitLab CI/CD .gitlab-ci.yml configuration for Gradle (Java / JDK 17) projects. With Docker build and push to repository container registry.

.gitlab-ci.yml

image: gradle:7.4-jdk17-alpine

stages:
    - build
    - test
    - packaging

# Disable the Gradle daemon for Continuous Integration servers as correctness
# is usually a priority over speed in CI environments. Using a fresh
# runtime for each build is more reliable since the runtime is completely
# isolated from any previous builds.
variables:
  GRADLE_OPTS: "-Dorg.gradle.daemon=false"

before_script:
  - GRADLE_USER_HOME="$(pwd)/.gradle"
  - export GRADLE_USER_HOME

build:
  stage: build
  script: gradle --build-cache assemble
  artifacts:
    paths:
      - build/distributions/*
  cache:
    key: "$CI_COMMIT_REF_NAME"
    policy: push
    paths:
      - build
      - .gradle

test:
  stage: test
  script: gradle check
  cache:
    key: "$CI_COMMIT_REF_NAME"
    policy: pull
    paths:
      - build
      - .gradle

docker-build:
  image: docker:latest
  stage: packaging
  services:
    - docker:dind
  before_script:
    # Credentials are automatically passed in by GitLab CI runner
    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
  # Default branch leaves tag empty (= latest tag)
  # All other branches are tagged with the escaped branch name (commit ref slug)
  script:
    - |
      if [[ "$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}"
  rules:
    - if: $CI_COMMIT_BRANCH
      exists:
        - Dockerfile