Enable using a custom docker client and network for E2E tests

[chatton]

Summary

For more involved tests, it can be necessary to perform more low level docker operations. E.g. in the rollkit feature branch, in order to create standalone celestia container, it is also required to ensure the remainder of the containers in chainA and chainB are also deployed on the same docker network.

I did a quick implementation here https://github.com/cosmos/ibc-go/blob/8152c32c0b54704d81919296965af64ba8eee93f/e2e/testsuite/testsuite.go#L366-L370 to get the E2E working, this implementation may be sufficient, we just need to add a bit of validation to the network and DockerClient fields, you must specify either both or neither for this to be correct.

This validation is not currently implemented on the linked branch.

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