A Travis CI configuration demo, it can auto build the Docker image, push it, release the JAR inside the image to GitHub Releases and tag it.

.travis.yml

language: java
services:
- docker
before_install:
- echo "$DOCKER_PASSWORD" | docker login "$DOCKER_REGISTRY" -u "$DOCKER_USERNAME" --password-stdin
- ./gradlew exportVersion
- export VERSION=$(cat version.txt)
- export VERSION_CODE=$(cat version_code.txt)
script:
- docker build -t "$DOCKER_REGISTRY"/"$DOCKER_IMAGE_NAME":"$VERSION" .
before_deploy:
  # ref: https://github.com/travis-ci/travis-ci/issues/2570#issuecomment-171262181
  - >
    if ! [ "$BEFORE_DEPLOY_RUN" ]; then
      export BEFORE_DEPLOY_RUN=1;
      # Extract JAR for deploying. ref: https://stackoverflow.com/a/31316636/6792243
      id=$(docker create "$DOCKER_REGISTRY"/"$DOCKER_IMAGE_NAME":"$VERSION")
      docker cp $id:/app/server.jar - > ./server.tar
      tar xvf server.tar
      mv server.jar server-$VERSION.jar
      docker rm -v $id
      git tag $VERSION
    fi
deploy:
  - provider: script
    script: docker push "$DOCKER_REGISTRY"/"$DOCKER_IMAGE_NAME":"$VERSION"
    on:
      branch: master
  - provider: releases
    name: $VERSION
    prerelease: true
    skip_cleanup: true
    api_key:
      secure: 2FwtavD4qGWijgsjp21hs45t1WFNsEMVlGQZMYnc0vwsehqwQIoVANJAARofAOQFG/UOJoYW413cozqkn7W68JrInVCejO78FZqhyvuChWt/Wp84+a+7894zXD0Cnthjx0dv+Sx4MOzFwp4xAMYLNlTWFAbmsW60e2cbM/HHsPjLbb55tCCxMIdEjG2jqgUu4Em3nJRLfFu1aol9D35vGqDx+E2tNZT8sBbWDe6NoTyAQsIiQe+viVxF5k8B1I8OEfqgJkc5V5YaPSq9KWbv9zTWdL/LZGbI8kCDve1RvZV48g9TRwVqz91wjKcHByvqBlO9jc6s70rpbgqF39Mx0N6fFHXZIDYMNY0l7wyO8RXAr6trV37wxGyptzsJhRJdf/CCb7hiJziLWAfBbFq6/2vSZnoUERnv42ef4dlxVfkA8yqBPeXGa8Ev4UCIa3ObfSYXUW8O3NtSwasLEsC0iMoHRvE4m8AQhHhvVP6h1Q7jsroiJdnIZN2FwF1sBHVOz5V1141HXMzlM9vc+kYzdTAIC8er+h4CE2133FNctlV2mlhLVBOXDx3kcH+algFwEPyWAR+a+g+DoegqavfABB36+SL9bz4vuohM3kY8Lbl5PbF5N2zV0sgSidRS234tTzToI5dt+yun5RUqm0CfgiEYpuyOc3PSvmKOmI9kiIA=
    file:
      - ./server-$VERSION.jar
    on:
      repo: Trumeet/PLS