01- Create two different docker containers (there are bitcoin.conf examples below) bitcoind-regtest-01 and bitcoind-regtest-02
pointing to different data folders (The regtest network will only work with at least 2 nodes).
bitcoind -conf=/path/to/bitcoind-regtest-01/bitcoin.conf -datadir=/path/to/bitcoind-regtest-01/data/folder -daemonbitcoind -conf=/path/to/bitcoind-regtest-02/bitcoin.conf -datadir=/path/to/bitcoind-regtest-02/data/folder -daemon
02- Create a docker network with the 2 containers
docker network create bitcoind-regtest-networkdocker network connect bitcoind-regtest-network bitcoind-regtest-01docker network connect bitcoind-regtest-network bitcoind-regtest-02