The client configs no longer stored in the test repo, but deployed by Retesteth itself to the folder ~/.retesteth inside the docker container

To build retesteth docker image you use the following docker image instructions:
(tested on Docker version 18.03.1-ce, build 9ee9f40)

1) Build the docker image https://github.com/ethereum/retesteth/blob/master/Dockerfile

2) Clone the test repo https://github.com/ethereum/tests

3) Edit config file for the retesteth to start your client ~/.retesteth/aleth/aleth.sh
Aleth is default client. just change the script line

aleth --test --db-path  --ipcpath  --log-verbosity 5 &

to

/tests/<yourClientName> --test --db-path  --ipcpath  --log-verbosity 5 &

the options --test --db--path and so on might be different in your client

4) Put your client executable to the test folder you have just cloned. /tests/<yourClientName>

4.2) Or run your client with TCP socket for RPC requests and configure reteseth config for it.
https://github.com/ethereum/retesteth/wiki/Add-client-configuration-to-Retesteth
Use --clients "<clientConfigFolderName>" option to run retesteth using your configuration.
Configurations folder: ~/.retesteth

5) To run the docker image

docker  run  -v "/localpath/to/test/repo:/tests"  <imageid> -t BlockchainTests -- --testpath /tests

(to get the recent imageid in docker run docker images)