Replace Bash builds with Gradle

[phreed]

The main motivation is to allow building on Windows.
bash is flaky on Windows.
Gradle works across platforms and has Docker plugins.

The affected files are:

• build-images.sh
• push-images.sh
• test-image.sh
• update.sh

The scripts used by the container would not be modified, i.e.

• docker-entrypoint.sh
• load-initialdb.sh

The recommended tool for building a container for a java application is

• https://github.com/GoogleContainerTools/jib

Other candidate plugins

• https://github.com/palantir/gradle-docker : require that docker be installed.
• https://github.com/bmuschko/gradle-docker-plugin : uses the docker remote api.