Document steps to produce a deterministic programs build

[Kristijan Rebernisak]

@archseer suggests building the program using this docker image:

docker run --rm -v "$(pwd)":/code \
  --mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
  --mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
  cosmwasm/workspace-optimizer:0.12.4

This is actually documented by contracts/ocr2/docs/Developing.md but not easily discoverable.

We should consider polishing the programs doc to have this more easily discoverable, and potentially even automate this step using a Makefile.