Print env vars resolution summary at the start of a deployment

[raphaeltm]

It is often the case that, when deploying, we expect an env var to be one value, but it is actually another. When we start a deployment, can we print a summary of all of the env vars how they are derived?

For example:

    environment:
      DATABASE_URL: postgresql://postgres:${POSTGRES_PASSWORD}@postgres:5432/postgres?sslmode=${SSL_MODE:-disable}

By default, for SSL_MODE, we should see something like "resolved to disable from default".
If SSL_MODE is set to require using defang config set SSL_MODE=require, we should see something like "resolved to *** resolved from config"

[lionello]

Our "run-time" (ie. in CD task) interpolation does not support any of the bash modifiers, so the default gets applied immediately. I started working on implementing this, but tabled it because I thought it wasn't important. I have a stash™.