I've been hacking on some very minor changes to the bookinfo sample app in an attempt to resolve #7047.

In doing so, I've discovered that the bookinfo build is not deterministic / repeatable for at least two reasons-- there may be others, but these are the two I was able to identify and quickly correct.

1. The first is that the productinfo flask app doesn't specify versions of its dependencies in its requirements.txt. This means that a build automatically gets the latest version of each package listed in that file. There are breaking changes in the latest version of at least one of those packages that break compatibility with the productinfo app.

2. The second is that the niaquinto/gradle Docker image used to facilitate the build of the reviews service service no longer exists.