chore: Add Gitlab CI/CD config

.gitlab-ci.yml

+image: openjdk:11-jdk
+stages:
+  - build
+  - test
+  - deploy
+
+variables:
+  MAVEN_CLI_OPTS: "-s ci_settings.xml --batch-mode"
+  MAVEN_OPTS: "-Dmaven.repo.local=.m2/repository -Dmaven.artifact.threads=10"
+  MAIN_BRANCH: "main"
+  RELEASE_BRANCH: "release"
+
+cache:
+  paths:
+    - .m2/repository/
+
+build:
+  stage: build
+  script:
+    - ./mvnw $MAVEN_CLI_OPTS $MAVEN_OPTS package
+  tags:
+    - vips-java
+
+test:
+  stage: test
+  script:
+    - ./mvnw $MAVEN_CLI_OPTS $MAVEN_OPTS test
+  tags:
+    - vips-java
+
+deploy-snapshot:
+  stage: deploy
+  script:
+    - ./mvnw $MAVEN_CLI_OPTS $MAVEN_OPTS deploy -DskipTests
+  tags:
+    - vips-java
+  rules:
+    - if: '$CI_COMMIT_REF_NAME == $MAIN_BRANCH'
+
+deploy-release:
+  stage: deploy
+  script:
+    # For debugging git interaction
+    #- export GIT_TRACING=2
+    #- export GIT_CURL_VERBOSE=1
+    - git config --global user.name "${GITLAB_USER_NAME}"
+    - git config --global user.email "${GITLAB_USER_EMAIL}"
+
+    # Use access token given by CI/CD variable ACCESS_TOKEN to authenticate
+    - export NEW_REPO_URL=https://root:$ACCESS_TOKEN@$CI_SERVER_HOST/$CI_PROJECT_PATH.git
+    - git remote set-url --push origin $NEW_REPO_URL
+    - git fetch origin --prune --prune-tags
+
+    # Checkout release branch, set release version and deploy. Commit, tag and push.
+    - git checkout $RELEASE_BRANCH
+    - git status
+    - git reset --hard origin/$RELEASE_BRANCH
+    - ./mvnw $MAVEN_CLI_OPTS $MAVEN_OPTS validate -DremoveSnapshot
+    - ./mvnw $MAVEN_CLI_OPTS $MAVEN_OPTS deploy -DskipTests
+    - export RELEASE_VERSION=$(mvn --batch-mode --no-transfer-progress --non-recursive help:evaluate -Dexpression=project.version | grep -v "\[.*")
+    - export RELEASE_TAG="v${RELEASE_VERSION}"
+    - git add pom.xml
+    - git commit -m "[ci skip] Set release version ${RELEASE_VERSION}"
+    - git tag -a $RELEASE_TAG -m "Tag release ${RELEASE_TAG}"
+    - git push origin $RELEASE_TAG
+    - git push origin $RELEASE_BRANCH
+
+    # Checkout main branch, merge release branch. Push commit.
+    - git fetch
+    - git checkout $MAIN_BRANCH
+    - git reset --hard origin/$MAIN_BRANCH
+    - git merge -m "[ci skip] Merge ${RELEASE_BRANCH}" --no-ff $RELEASE_BRANCH
+    - git push origin $MAIN_BRANCH
+  tags:
+    - vips-java
+  rules:
+    - if: '$CI_COMMIT_REF_NAME == $RELEASE_BRANCH'

ci_settings.xml

+<settings xmlns="http://maven.apache.org/SETTINGS/1.1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+          xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.1.0 http://maven.apache.org/xsd/settings-1.1.0.xsd">
+    <servers>
+        <server>
+            <id>gitlab-maven</id>
+            <configuration>
+                <httpHeaders>
+                    <property>
+                        <name>Job-Token</name>
+                        <value>${CI_JOB_TOKEN}</value>
+                    </property>
+                </httpHeaders>
+            </configuration>
+        </server>
+    </servers>
+</settings>
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