Update Docker Image
- Don't add git ignore or ci script files to the docker image.
- Use a whitelist format for the git ignore file.
- Update Gradle version.
- Install Gradle files under /opt rather than /usr/share as recommended by the official documentation.
- Remove all quiet flags and use the expanded form of other flags.
- Add a health check function to the image.
- Specify environment variables as a single operation in order to save space and time.