Figure out nicer way to pre-install dependencies in Docker

Currently, we pre-install the dependencies for offline usage in the Dockerfile as follows:

```dockerfile
COPY src/ src/
COPY build.gradle .
RUN gradle build
```

Basically, we have a "project" with two empty source files: https://github.com/exercism/groovy-test-runner/blob/main/src/main/groovy/TestRunner.groovy and https://github.com/exercism/groovy-test-runner/blob/main/src/test/groovy/TestRunnerSpec.groovy and a `build.gradle` file: https://github.com/exercism/groovy-test-runner/blob/main/build.gradle. Compiling this project forces the dependencies to be downloaded, which can then be used later on to run the student's solution without any networking (which is how test runners are executed).

While this works, it does feel a bit odd. Maybe there is a nicer solution?