The web application is a Maven project in the webapp directory of the repo. It should be imported into Eclipse or your preferred Java IDE. Instructions for Eclipse are covered in Running from Eclipse.
Running the command
mvn post-site will generate project documentation into the target folder and copy it across to the GitHub Pages site in the docs directory of the repo.
The application camn also be built as a Docker image locally, using Google jib. You will need Docker installed and Maven 3.5 or newer. It is built using the command
mvn compile jib:dockerBuild. For more information on using jib to build a Docker image locally, see https://cloud.google.com/java/getting-started/jib#build-jib.
Alternatively, you can build using jib to deploy to a private repository, using
mvn compile jib:build. You will need to amend the
<image> tag in the pom.xml to point to the relevant server.