Support changing `io.podman` label domain namespace

**Is your feature request related to a problem? Please describe.**

It is possible that we have two different processes where one needs to use `docker-compose` and the other `podman-compose` but interacting with the same container socket, containers, and compose project. For example, maybe certain configuration is only available in one or the other and no single implementation covers the entire use-case.

Unfortunately, containers started by one is not visible from the other.

This is due to project-matching being done by matching the label `io.podman.compose.project`. Containers started by docker-compose instead carry the `io.podman.compose.project`. It seems to be no way to make them work together due to this.

**Describe the solution you'd like**

Introduce a new environment variable like `COMPOSE_LABEL_NAMESPACE` defaulting to `io.podman`. Users can then interact with projects managed by docker-compose via `COMPOSE_LABEL_NAMESPACE=com.docker`, or set their own namespace.

**Describe alternatives you've considered**
Transparently filter for both currently known namespaces. This seems to not be worth it.

**Additional context**