Ensure mvn build step always runs

[rovats]

May be use a random number generator URL?

Before mvn clean package:

ADD randomURL /tmp/cachebust

[rovats]

quite tricky to bust docker cache only for specific instructions, we can use a url that generates random text, but that's probably not desirable (pinging out to a random url).

Probably best to use --no-cache docker option when running the docker build command if the build needs to be forced and the code hasn't changed (eg: if the database has been re-created).

In the normal case, the build will work fine as it uses Github api to figure out if code has changed, and builds the necessary parts again (mvn clean package will rerun if any file has changed in the repo).

[rovats]

Added force_build_timestamp build arg, which can be used to selectively bust docker cache (for specific instructions).