Prebuild shared base Docker image for agent containers to speed up ventis build

[Saaketh0]

All agent Dockerfiles independently form python:3.11-slim and run pip install on nearly identical requirements.txt files, redoing the same ~200MB dependency install on every build.

Create a shared ventis-base image with the common deps (grpcio, redis, pyyaml, ipdb, ipython) pushed to a registry (or baked into the AMI), and have per-agent Dockerfiles do FROM ventis-base:latest + COPY . . only.

Should cut per-agent build time.