misc: Make basics work in Docker container

Should now be able to run a basic introducer + storage node setup in
docker container.

Example:

    $ docker run --rm tahoe-lafs introducer
    # Copy introducer URL from stdout
    $ tests/run-storage-nodes.sh -i pb://nodeid@172.17.0.220:3455,127.0.0.1:3455/secret

Only works on internal docker network currently.