Connecting Mist – How to Connect Mist with Testrpc on a Mac?

Mist starts its own geth process, which uses the same default chaindata database location as the geth client you run manually. When you start the geth client first, it acquires a lock on the database files and prevents Mist from accessing them.

If you need to run both of them simultaneously, use a separate data directory for geth by starting it with --datadir option.

It is because you use loopback interface when trying to connect. Just try to use:

docker run --rm -it ethereum/client-go \
attach http://$(ip route \
|grep "default via" \
|grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}' \
|grep -m2 "" \
|tail -1):8545