[#1] Add a CI/CD pipeline to build the Gradle project in a Docker container

Associated epic: #1 (closed).

Upon push, a new JAR file should be compiled. This JAR file must be runnable on a Spigot server (this requires it to be a "fat" JAR, to include any dependencies). Compilation on any branch other than the main branch should use the -SNAPSHOT suffix.

The CI/CD process should be run from within a Docker container.