Consider single-container version of Superset for local devtest use cases

The current implementation takes a docker-compose.yml file which runs several containers.

Our initial spec in Add `container_spec` for containerized plugin c... (meltano#2205 - closed) will expect a single container definition.

Following from the alternate instructions at https://superset.apache.org/docs/installation/installing-superset-from-scratch, what do you think about us having a version of this that runs in a single container? Assumption being that the PostgresDB is launched in a separate process, and does not need to be part of the execution command.

In theory a bootstrap script in the docker container could import the file definitions and then export after execution completes (assuming it gets a graceful shutdown, or assuming ctrl+c is only hit once and enough time is allowed before a hard process kill).

@pnadolny13, @pandemicsyn - Curious as to your thoughts.