Add Docker Dual-Mode issue to Known Issues (#2090)

README.md

@@ -301,3 +301,7 @@ If genesis sync passed, then it's fine to run multiple Erigon on same Disk.
 
 Please read https://github.com/ledgerwatch/erigon/issues/1516#issuecomment-811958891
 In short: network-disks are bad for blocks execution - because blocks execution reading data from db non-parallel non-batched way.
+
+### rpcdaemon "Dual-Mode" does not work with Docker Container
+
+Running rpcdaemon in "Dual-Mode" (including the `--datadir` flag) generally results in better performance for RPC calls, however, this does not work when running erigon and rpcdaemon in separate containers. For the absolute best performance bare metal is recommended at this time.