Set up cache for multistage build

We build the container in a multi-stage build:

(1) JDK, build jar
(2) JRE, install runtime dependencies plus the jar

Unfortunately we don't store/cache the 1st phase. So all the dependency downloads etc don't get cached.

We'd need to:

1- split up the shadowJar command into installing dependencies, THEN building the shadowJar
2- build the build-container with a target, then upload that target to the repository

Then the runtime container would be built as usual, copying in the jar cached from the build phase, which would rebuild if the source or dependencies changed…