explicitly set i2p.dir.base and i2p.dir.config in router.config for Docker containers. fixes and closes #41.

docker/rootfs/i2p/router.config

@@ -2,3 +2,5 @@ router.updateDisabled=true
 i2np.ntcp.port=12345
 i2np.udp.port=12345
 i2np.udp.internalPort=12345
+i2p.dir.base=/i2p
+i2p.dir.config=/i2p/.i2p