Private IPFS cluster testing using Docker

For testing purposes i have created a cluster on a single machine using Docker.
Everything look ok so far. When i add a file to a cluster node it also gets pinned to others.

But from what i can see, although the cluster is private, the underlying ipfs nodes are not.
The cluster will show as pinned files only I have added, but the ipfs daemons keep connecting to a large number of outside nodes.
In my case i need my network to be completely isolated and i guess connecting to all these outsiders generate a huge overhead.

When dealing with normal ipfs setup i know that i need a bootstrap list with only my own nodes in it.
But how can i do the same using the docker-compose file?
For specific design reasons i cannot have any kind of persistency, so i cannot edit the permanent config of each node. It has to be done during the deployment of the official ipfs docker images.

For now i achieve this using scripts that auto remove all bootstraps and disconnect all peers just after the services start, but this is far from elegant.

So is there a way to remove all default bootstraps using docker-compose?
Also is there a way to alter the command compose executes, in order to include --enable-pubsub-experiment without rebuilding a new dockerfile?