First, generate a JSON file of 64 unencrypted private keys in a common directory:
mkdir /tmp/keys
bazel run //tools/unencrypted-keys-gen -- --num-keys 64 --output-json /tmp/keys/val_1.json
Then, run the Eth1-Mock-RPC server:
bazel run //:eth1-mock-rpc -- --genesis-deposits 64 --unencrypted-keys-dir /tmp/keys