Update testnet scripts to support builder testing and update README.md.

scripts/local_testnet/README.md

@@ -128,3 +128,16 @@ Update the genesis time to now using:
 
 > Note: you probably want to just rerun `./start_local_testnet.sh` to start over
 > but this is another option.
+
+### Testing builder flow
+
+1. Add builder URL to `BN_ARGS` in `./var.env`, e.g. `--builder http://localhost:8650`. Some mock builder server options: 
+    - [`mock-relay`](https://github.com/realbigsean/mock-relay)
+    - [`dummy-builder`](https://github.com/michaelsproul/dummy_builder)
+2. (Optional) Add `--always-prefer-builder-payload` to `BN_ARGS`.
+3. The above mock builders do not support non mainnet presets as of now, and will require setting `SECONDS_PER_SLOT` and `SECONDS_PER_ETH1_BLOCK` to `12` in `./vars.env`. 
+4. Start the testnet with the following command (the `-p` flag enables the validator client `--builder-proposals` flag:
+    ```bash
+    ./start_local_testnet -p genesis.json
+    ```
+5. Block production using builder flow will start at epoch 4.

scripts/local_testnet/beacon_node.sh

@@ -63,4 +63,5 @@ exec $lighthouse_binary \
 	--disable-packet-filter \
 	--target-peers $((BN_COUNT - 1)) \
   --execution-endpoint $execution_endpoint \
-  --execution-jwt $execution_jwt
+  --execution-jwt $execution_jwt \
+  $BN_ARGS

scripts/local_testnet/setup.sh

@@ -47,6 +47,6 @@ lcli \
 	insecure-validators \
 	--count $VALIDATOR_COUNT \
 	--base-dir $DATADIR \
-	--node-count $BN_COUNT
+	--node-count $VC_COUNT
 
 echo Validators generated with keystore passwords at $DATADIR.

scripts/local_testnet/vars.env

@@ -61,5 +61,8 @@ SECONDS_PER_ETH1_BLOCK=1
 # Proposer score boost percentage
 PROPOSER_SCORE_BOOST=40
 
+# Command line arguments for beacon node client
+BN_ARGS=""
+
 # Command line arguments for validator client
 VC_ARGS=""