docker image should be independent of host contract

[amessbee]

In example dApps Offer-up and dapp-agoric-basics, the docker container started by yarn start:docker command is specific to the dApp it is started in. One cannot start a different contract on a chain running in such a container. From developer experience point of view, it seems more reasonable that yarn start:docker only starts a docker container with a a3p instance running, and one should be able to deploy any of the example contracts on any such running container.

This is probably because of the limitations of how docker compose up works.