You can use environment variables to customize config (see docker run environment options):
docker run -v btcd-data:/btcd --name=btcd-node -d \
-p 8333:8333 \
-p 127.0.0.1:8334:8334 \
-e RPCUSER=mysecretrpcuser \
-e RPCPASS=mysecretrpcpass \
lnzap/btcd
Or you can use your very own config file like that:
docker run -v btcd-data:/btcd --name=btcd-node -d \
-p 8333:8333 \
-p 127.0.0.1:8334:8334 \
-v /etc/mybtcd.conf:/btcd/.btcd/btcd.conf \
lnzap/btcd